any app which gets your imagination going and you can keep building on for a while is a good app. You sound like you want a database challenge so, start with a simple db+client and then work to a 3 teir approach and get into those kinds of aspects.
Well, my "test" app is an RPG game a bit like a muck, so Im recreating an evironment I know well but I can extend it to the point of writing my own programming language with in it, multi user, networks, reports, db, I can make it as much or as little as I want.
The key part is to find something interesting - another one of mine is a "things to do " program, where you schedule things, or just have "stuff" you need to do, priorities, sorting, different class types for different "things" you're doing, eg, maybe a holiday which has a start and end, or just something you want to work on.. so has no start, no end, but is "on going".. it may have sub parts, some compulsary, some not..
or it could be a game like my RPG game.. it just needs to be something that you're interested, that you'd use, if you wouldnt use it, its pointless.