Go for the tablet using ONLY native Objective C. Don't use PhoneGap.

It will be a good learning experience, you will be able to instantly test the code, and there are no two ways about it. Few hacks or tricky stuff. It's a pretty straightforward platform.

Also, you can easily earn money with this, and the money is purely passive. No server administration, etc. You can finish it, put it in the store and run off for the next year.

Also, you learn about memory management and device constraints.

If you want something even more interesting, consider a use-case like using a tablet to control some hardware (like a lightbulb).