The Google Script will download your Fitbit data via the Fitbit API and insert it into a Google spreadsheet. The first row of the spreadsheet will be a header row containing data element names like steps walked, body fat, calories burned, etc. Subsequent rows will contain data, one day per row.

This is a Google Spreadsheet bound script so you need to create a sheet first and put this code inside the Script editor. Fitbit uses metric units (weight, distance) so you may wish to convert them as per your locale.

/*
Original Fitbit script by loghound@gmail.com,
Further modifications by Mark Leavitt, Christian Stade-Schuldt, Robert Furberg, Amit Agarwal
*/

// Key of ScriptProperty for Fitbit consumer key.
var CONSUMER_KEY_PROPERTY_NAME = 'fitbitConsumerKey';
// Key of ScriptProperty for Fitbit consumer secret.
var CONSUMER_SECRET_PROPERTY_NAME = 'fitbitConsumerSecret';
// Default loggable resources (from Fitbit API docs).
var LOGGABLES = [
  'activities/steps',
  'activities/distance',
  'activities/floors',
  'activities/elevation',
  'activities/calories',
  'activities/activityCalories',
  'activities/minutesSedentary',
  'activities/minutesLightlyActive',
  'activities/minutesFairlyActive',
  'activities/minutesVeryActive',
  'sleep/startTime',
  'sleep/timeInBed',
  'sleep/minutesAsleep',
  'sleep/awakeningsCount',
  'sleep/minutesAwake',
  'sleep/minutesToFallAsleep',
  'sleep/minutesAfterWakeup',
  'sleep/efficiency',
  'body/weight',
  'body/bmi',
  'body/fat',
];

// function authorize() makes a call to the Fitbit API to fetch the user profile
function authorize() {
  var oAuthConfig = UrlFetchApp.addOAuthService('fitbit');
  oAuthConfig.setAccessTokenUrl('https://api.fitbit.com/oauth/access_token');
  oAuthConfig.setRequestTokenUrl('https://api.fitbit.com/oauth/request_token');
  oAuthConfig.setAuthorizationUrl('https://api.fitbit.com/oauth/authorize');
  oAuthConfig.setConsumerKey(getConsumerKey());
  oAuthConfig.setConsumerSecret(getConsumerSecret());
  var options = {
    oAuthServiceName: 'fitbit',
    oAuthUseToken: 'always',
  };
  // get the profile to force authentication
  Logger.log('Function authorize() is attempting a fetch...');
  try {
    var result = UrlFetchApp.fetch('https://api.fitbit.com/1/user/-/profile.json', options);
    var o = Utilities.jsonParse(result.getContentText());
    return o.user;
  } catch (exception) {
    Logger.log(exception);
    Browser.msgBox('Error attempting authorization');
    return null;
  }
}

// function setup accepts and stores the Consumer Key, Consumer Secret, firstDate, and list of Data Elements
function setup() {
  var doc = SpreadsheetApp.getActiveSpreadsheet();
  var app = UiApp.createApplication().setTitle('Setup Fitbit Download');
  app.setStyleAttribute('padding', '10px');

  var consumerKeyLabel = app.createLabel('Fitbit OAuth Consumer Key:*');
  var consumerKey = app.createTextBox();
  consumerKey.setName('consumerKey');
  consumerKey.setWidth('100%');
  consumerKey.setText(getConsumerKey());
  var consumerSecretLabel = app.createLabel('Fitbit OAuth Consumer Secret:*');
  var consumerSecret = app.createTextBox();
  consumerSecret.setName('consumerSecret');
  consumerSecret.setWidth('100%');
  consumerSecret.setText(getConsumerSecret());
  var firstDate = app.createTextBox().setId('firstDate').setName('firstDate');
  firstDate.setName('firstDate');
  firstDate.setWidth('100%');
  firstDate.setText(getFirstDate());

  // add listbox to select data elements
  var loggables = app.createListBox(true).setId('loggables').setName('loggables');
  loggables.setVisibleItemCount(4);
  // add all possible elements (in array LOGGABLES)
  var logIndex = 0;
  for (var resource in LOGGABLES) {
    loggables.addItem(LOGGABLES[resource]);
    // check if this resource is in the getLoggables list
    if (getLoggables().indexOf(LOGGABLES[resource]) > -1) {
      // if so, pre-select it
      loggables.setItemSelected(logIndex, true);
    }
    logIndex++;
  }
  // create the save handler and button
  var saveHandler = app.createServerClickHandler('saveSetup');
  var saveButton = app.createButton('Save Setup', saveHandler);

  // put the controls in a grid
  var listPanel = app.createGrid(6, 3);
  listPanel.setWidget(1, 0, consumerKeyLabel);
  listPanel.setWidget(1, 1, consumerKey);
  listPanel.setWidget(2, 0, consumerSecretLabel);
  listPanel.setWidget(2, 1, consumerSecret);
  listPanel.setWidget(3, 0, app.createLabel(' * (obtain these at dev.fitbit.com)'));
  listPanel.setWidget(4, 0, app.createLabel('Start Date for download (yyyy-mm-dd)'));
  listPanel.setWidget(4, 1, firstDate);
  listPanel.setWidget(5, 0, app.createLabel('Data Elements to download:'));
  listPanel.setWidget(5, 1, loggables);
  // Ensure that all controls in the grid are handled
  saveHandler.addCallbackElement(listPanel);
  // Build a FlowPanel, adding the grid and the save button
  var dialogPanel = app.createFlowPanel();
  dialogPanel.add(listPanel);
  dialogPanel.add(saveButton);
  app.add(dialogPanel);
  doc.show(app);
}

// function sync() is called to download all desired data from Fitbit API to the spreadsheet
function sync() {
  // if the user has never performed setup, do it now
  if (!isConfigured()) {
    setup();
    return;
  }

  var user = authorize();
  var doc = SpreadsheetApp.getActiveSpreadsheet();
  doc.setFrozenRows(1);
  var options = {
    oAuthServiceName: 'fitbit',
    oAuthUseToken: 'always',
    method: 'GET',
  };
  // prepare and format today's date, and a list of desired data elements
  var dateString = formatToday();
  var activities = getLoggables();
  // for each data element, fetch a list beginning from the firstDate, ending with today
  for (var activity in activities) {
    var currentActivity = activities[activity];
    try {
      var result = UrlFetchApp.fetch(
        'https://api.fitbit.com/1/user/-/' + currentActivity + '/date/' + getFirstDate() + '/' + dateString + '.json',
        options
      );
    } catch (exception) {
      Logger.log(exception);
      Browser.msgBox('Error downloading ' + currentActivity);
    }
    var o = Utilities.jsonParse(result.getContentText());

    // set title
    var titleCell = doc.getRange('a1');
    titleCell.setValue('date');
    var cell = doc.getRange('a2');

    // fill the spreadsheet with the data
    var index = 0;
    for (var i in o) {
      // set title for this column
      var title = i.substring(i.lastIndexOf('-') + 1);
      titleCell.offset(0, 1 + activity * 1.0).setValue(title);

      var row = o[i];
      for (var j in row) {
        var val = row[j];
        cell.offset(index, 0).setValue(val['dateTime']);
        // set the date index
        cell.offset(index, 1 + activity * 1.0).setValue(val['value']);
        // set the value index index
        index++;
      }
    }
  }
}

function isConfigured() {
  return getConsumerKey() != '' && getConsumerSecret() != '';
}

function setConsumerKey(key) {
  ScriptProperties.setProperty(CONSUMER_KEY_PROPERTY_NAME, key);
}

function getConsumerKey() {
  var key = ScriptProperties.getProperty(CONSUMER_KEY_PROPERTY_NAME);
  if (key == null) {
    key = '';
  }
  return key;
}

function setLoggables(loggable) {
  ScriptProperties.setProperty('loggables', loggable);
}

function getLoggables() {
  var loggable = ScriptProperties.getProperty('loggables');
  if (loggable == null) {
    loggable = LOGGABLES;
  } else {
    loggable = loggable.split(',');
  }
  return loggable;
}

function setFirstDate(firstDate) {
  ScriptProperties.setProperty('firstDate', firstDate);
}

function getFirstDate() {
  var firstDate = ScriptProperties.getProperty('firstDate');
  if (firstDate == null) {
    firstDate = '2012-01-01';
  }
  return firstDate;
}

function formatToday() {
  var todayDate = new Date();
  return (
    todayDate.getFullYear() +
    '-' +
    ('00' + (todayDate.getMonth() + 1)).slice(-2) +
    '-' +
    ('00' + todayDate.getDate()).slice(-2)
  );
}

function setConsumerSecret(secret) {
  ScriptProperties.setProperty(CONSUMER_SECRET_PROPERTY_NAME, secret);
}

function getConsumerSecret() {
  var secret = ScriptProperties.getProperty(CONSUMER_SECRET_PROPERTY_NAME);
  if (secret == null) {
    secret = '';
  }
  return secret;
}

// function saveSetup saves the setup params from the UI
function saveSetup(e) {
  setConsumerKey(e.parameter.consumerKey);
  setConsumerSecret(e.parameter.consumerSecret);
  setLoggables(e.parameter.loggables);
  setFirstDate(e.parameter.firstDate);
  var app = UiApp.getActiveApplication();
  app.close();
  return app;
}

// function onOpen is called when the spreadsheet is opened; adds the Fitbit menu
function onOpen() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var menuEntries = [
    {
      name: 'Sync',
      functionName: 'sync',
    },
    {
      name: 'Setup',
      functionName: 'setup',
    },
    {
      name: 'Authorize',
      functionName: 'authorize',
    },
  ];
  ss.addMenu('Fitbit', menuEntries);
}

// function onInstall is called when the script is installed (obsolete?)
function onInstall() {
  onOpen();
}