I used to wail out singles and paradiddles. Nowadays I get some of the boring practice of harder rudiments out of the way by playing them slowly and perfectly, paying attention to timing and volume. I also try to incorporate my feet, work on independence and the such. Perfect thing to do during Spanish class, haha.

Make whipped cream with whipping cream and sugar. The effort required to whip it is perfect for building fingers, wrists, and forearms. And when you're done, you have a tasty treat.

working on my solo for solo and ensamble in study hall has done a lot for my familiarity for the piece because i am able to work out all of the stickings and rhythms on my desk (the piece is two toms and two timbales) but by the time seventh period percussion class comes around, i have reviewed the whole piece enough that i only need to work on a couple parts that deal with coordinating my hands instead of spending all my free practice time figuring out stickings and rhythms.

Usually in my 7th grade Math class my friend darius (quads) and i (snare) will make upa cadence and get in soo much trouble

Im one of those that practices my rudiments with my feet. I find it funny now because I will naturally do it and later realize Im doing it.

This might sound weird, but when I don't have sticks, i'll do one hand clapping exercises. Basically the objective is to clap as loud as you can using only one hand. I'll do one hand claps in each hand for as long as I can until I have to stop. You really want to try to go for the burn. I wouldn't really consider it "practicing" but it's definitely a great warm up or chop builder. I'll do it sometimes when I have to go play out of the blue without getting a proper warm up.

I take the train and travel in general quite a bit with other people around so it is hard to even use a quiet pad and sometimes even sticks on my legs. However, it doesn't stop me. We are limited by sticks when we don't need to be. Why do you need sticks to do a timing exercise? You don't!

So, take your metronome and plug your headphones into it and count quietly under your breath, just loud enough for you, and play your timing exercises. Try lots of different clicks and counting schemes. Or play your double beat with your metronome. Really anything. Of course you won't be able to play exercises that use bounce or rebound but there are loads of exercises dedicated to counting or inner pulse etc...etc... Go over to the exercise section here or snarescience for example and find an exercise that doesn't rely on rebound and develop your counting skills or speed etc...