I come up with many ideas for iPad apps. I have almost zero time to write them, but I do come up with ideas every couple weeks. And when I take my vacation time, I'll write one. How to choose.
Well here's an idea. Choose the hardest one you can write in the time allowed, assuming you think it will sell. Hard means people will be agreeable to paying $5 for an app. Hard means you won't have cut rate competition the next week. Hard means you can be proud of showing what you are capable of.
iOS is filled with programming tasks that should be hard but aren't. Want to play a H.264 MP4 movie, it's like 5 lines of code. Want to play streaming broadcast MPEG2 off your HDHomerun, well that's hard (and probably involves getting an expensive license from Dolby for the sound amongst other IP fees) to do without hardware decoding. Nobody will pay you for the former, there are people who will pay for the latter. Then again, the latter might also be impossible on even an A5.
So I have my own hard idea, which I'm not sharing, and I'll be spending the summer writing it. I'll let you know how it turns out.