hmm, well i can think of a few:
1/ lie on your back, with your legs at right angles in the air. Then get someone to stand with one foot either side of your head and gently push your legs away. obviously you resist the push and bring your legs back to the origional position.... that ones damn hardcore
2/ jump on one of the leg machiens that can be found on the edge of your average bench, there are a few exercises to do here.
3/ run up something steep, like stairs are good.
4/ go into press-up position, jump yous legs up, so they touch your chest (keep your hands on the floor), and then jump back to origional position, then repeat.
that should keep you busy for a while