This script fetches stuff from the fitbit api. You need to register an app (for consumer key and secret) and authorize this script on behalf of your account. One way to do this is with the out of band authentication. Feel free to expand this script to automate this.

fitbit_archiver.rb

# encoding: UTF-8

require 'rubygems'
require 'date'
require 'optparse'
require 'ostruct'
require 'oauth'
require 'json'

class FitbitArchiver
  attr_accessor :oauth, :day, :archive
  
  def initialize(options)
    consumerKey = "<<YOUR_API_KEY>>"
    consumerSecret = "<<YOUR_API_SECRET>>"
    michaelKey = "<<YOUR_KEY_AFTER_AUTHORIZING_THIS_SCRIPT>>"
    michaelSecret = "<<YOUR_SECRET_AFTER_AUTHORIZING_THIS_SCRIPT>>"
    
    @oauth = OAuth::AccessToken.new OAuth::Consumer.new(consumerKey, consumerSecret), michaelKey, michaelSecret
    @day = (options.day ? options.day : Date.today - 1).strftime '%Y-%m-%d'
    @archive = "#{File.expand_path(File.dirname(__FILE__))}/archive"
  end
  
  def archive_body
    data = retrieve_json format("http://api.fitbit.com/%s/user/-/body/date/%s.json", 1, day)
    write_to_file __method__.to_s.gsub('archive_', ''), data
  end

  def archive_body_weight
    data = retrieve_json format("http://api.fitbit.com/%s/user/-/body/log/weight/date/%s.json", 1, day)
    write_to_file __method__.to_s.gsub('archive_', ''), data
  end

  def archive_body_fat
    data = retrieve_json format("http://api.fitbit.com/%s/user/-/body/log/fat/date/%s.json", 1, day)
    write_to_file __method__.to_s.gsub('archive_', ''), data
  end

  def archive_activities
    data = retrieve_json format("http://api.fitbit.com/%s/user/-/activities/date/%s.json", 1, day)
    write_to_file __method__.to_s.gsub('archive_', ''), data
  end

  def archive_sleep
    data = retrieve_json format("http://api.fitbit.com/%s/user/-/sleep/date/%s.json", 1, day)
    write_to_file __method__.to_s.gsub('archive_', ''), data
  end

  private
    def retrieve_json(url)
      response = oauth.get url
      case response
        when Net::HTTPSuccess then JSON.parse(response.body)
        else nil
      end    
    end
    
    def write_to_file(type, data)
      File.open("#{archive}/#{type}.#{day}.json", 'w') {|f| f.write(JSON.pretty_generate(data)) } if data
    end
end


begin
  options = OpenStruct.new
  
  ARGV.options do |opts|
    opts.on("-dDAY", "--day DAY", "Use the given date instead of the day before  today") do |day|
      options.day = Date.parse day
    end
    
    opts.on_tail("-h", "--help", "Show this message") do
      puts opts
      exit
    end    
  end

  ARGV.parse!
  fitbitArchiver = FitbitArchiver.new options
  fitbitArchiver.methods.grep(/archive_.*/).each{|method| fitbitArchiver.send method}
end