The Processing Discourse is the first place to start your research. Here, people exchange ideas and techniques used in creating interactive apps with Processing. You can post your own questions to the forum, and more often than not get detailed, supportive responses:
http://processing.org/discourse/yabb/YaBB.cgi?board=Contribution_Responsive
Musical Apps
Musical applications are very similar to games, although (usually) there's no score, and it's more about playing with the app rather than trying to "win". Here are some examples of musical apps:
This is an example of a musical sequencer by Lukas Vojir. Individual objects are placed on the canvas that each have a sound property that plays when the playhead crosses them. From
I started with an idea of placing objects on a stage and having a ball bouncing from them - and playing different samples.. it was quite fun, but the sound output was random too much.. even too much for me.. so I brought it back to relatively “boring” from-left-to-right-going-playhead concept..via: Lukas Vojir
Simple Games
Side-Scrolling games are a simple structure to base a project on, but there are many different side-scrolling approaches.
A simple "tracking" game.
http://openprocessing.org/visuals/?visualID=16238
A bunch of different games uploaded on OpenProcessing.org
http://openprocessing.org/tools/search.php?q=game
Games by NMcCoy. Most of his games include the source code for reference:
Snake Oil is a cross between Asteroids and the classic snake game.
Neutronium Verge is an interesting puzzle game.
Wave Spark is an interesting side-scrolling game. It’s a simple premise – use one button to control gravity in a hilly environment, and go as fast as you can.
Using Open Source Projects as Reference
Looking at software that's been made open source is an excellent way to learn about coding different aspects of interactive applications. Games are especially interesting because they are made to be played with and enjoyed, so they are programmed with the end user in mind. Musical Apps are similar, as they are meant to be played with as a musical instrument. There is are distinct overlaps between the two. There is a lot to learn from programming simple game or music concepts.
More Inspiration
Jumping Game:
