Every tuesday and thursday I gotta go to gymnastics practice and if you want a huge work out, that's where you can find it.
Find an open gym, where kids usually go on the little trampolines and all that and work the other equipment.
High Bar can be just chin ups and pull ups.
Parallel Bars, you can do dips, which is when you grab each bar (they're like this --> = ), and you "dip" up and down. 30 of these is usually a good target.
You can also go on the rings and do pull-ups on that, twisting the rings from out to in each time. Those are pretty tough.
You could also do handstand touches against a wall in which you touch your hand to your shoulder and alternate hands (do this 100 times or so).
Don't forget to work the core, too! Crunches are sorta helpful.