Fly around, collect weapons, and shoot at things. It doesn't get much more simple than that. Well, I suppose just standing and shooting, or simply standing, but that wouldn't be fun, now would it? If that's too simple for you, go ahead and create your own levels, placing enemies and power-ups on various bits of scenery. Once created you can share your levels on the Jetpack Attack website, or via many popular social networking dealie-o's.
As far as flash games go, it's certainly a way to waste many minutes of your time. Check it out by clicking on the link I've hidden cleverly in the text below.
Dark Void presents Jetpack Attack [Flash Site]