Add an extra level of security to your application with client side encryption. Even over an SSL connection, there are attacks which could expose your users' sensitive information. Using JSEncrypt, learn how to encrypt on the client side and decrypt on the server side.

Using the Google Maps Javascript API, learn how to plot markers onto a map. Load markers and info windows through AJAX calls to your Rails JSON API.

Using Bootstrap in your application has become much more simple. Also learn to create some helper methods to make working with Bootstrap much easier.

Using the Geocoder gem and Google APIs, learn how to add some geolocation functionality to your application.

Mountable Engines are a great way to extract code into its own namespace and allow the code to be reused in other applications. Other popular gems that are mountable engines are Devise and Doorkeeper.