Skip the donuts man
Try some bicycles (where you are lying on your back and you bring your knee into the opposite elbow, then do the other side). Alternate back and forth for 20-35 reps per side. If it is too easy then slow the movement down a tad. I'm sure you've seen the move before. Also, you would probably get better results doing your leg raises while hanging from a chin-up bar.
Cheers, GTForce