for those of you who might be a bit inexperienced, please don't try either of the first two exercises, without having a knowledgeable coach/instructor present, as these can be a bit dangerous if done incorrectly.
"burpees" are great, but beyond many beginners' capabilitites as well. you can modify them to make them a bit easier (yet still challenging) by removing both the pushup component, and the end-jump component. when i first started trying these, that's what i had to do. basically, they become squat-thrusts.
from a standing position, squat down until your hands (palms) are touching the floor, closeto/directly next to your feet/shoes. at this point, you kick both feet out behind you, so that your approximately in the beginning position of a conventional pushup. pull both feet back up to your hands (the original squat position), then stand up. that's one repetition.
try to start with three sets of 10 squat-thrusts. when they get easy, add reps or sets, or add in a jump at the end (rather than just standing up, you'll try to jump as high as you can, and immediately upon landing, drop to the squat position again, and repeat.) once that's easy, add in the pushup. (when you kick your feet out behind you, lower your body at the same time, with your arms, so that when your feet are all the way back, you're in a "down" pushup position.) when you pull your feet back up to your hands, you should also push yourself up to a straight-arm position, then stand/jump again.
this can be a really fun, simple, and *quick* exercise, and it can be done virtually anywhere, at almost any time!