Google codelabs provide guided, tutorial, hands-on coding experiences. Most codelabs will step you through the process of building a small application, or adding a new feature to an existing application. They cover a wide range of topics such as Android Wear, Google Compute Engine, Project Tango, and Google APIs on iOS.
You can access Google codelabs on the web at g.co/codelabs. You should be able to complete any of them in your favorite development environment. Just pick a codelab about a topic that you find interesting and follow the instructions.