In the past I've mostly grown tomatoes in the greenhouse. I've had peppers in the greenhouse in the past, but they seem to attract whiteflies more than the tomatoes, and it's hard to kill them when there's fish in the greenhouse. You have to be careful what kind of bug killers you use because most of them will also kill fish.
Last year, I grew a couple of watermelon plants in the greenhouse and that was pretty successful so I'm going to do that again this year. I'll probably try a few other things also, but I don't know what yet.