I really like simple games, which you can play where ever you are - and which still has enough challenges to be interesting for quite a while. One of the best games I’ve played for a long time is Flight Control for the iPhone/iPod Touch. The game was developed by Firemint. It’s a fairly simple game. You’re in control of an Airport and all you have to do is make sure the planes land at the runways.
Working with application development - either on the web, on the desktop or any other place - is often quite interesting. When making new releases features are added, changed - or in rare cases removed. As a developer - or “software product manager” - it must be an interesting challenge to keep up with the users and the market to capture the features and changes to a product, which will make it better from release to release.