I like this exercise because it engages your entire core. And it's hard (so I feel like I'm doing something). I actually do this one while holding an exercise ball- so you have to touch it to the ground on either side.
Basically you sit on the floor, knees bent, heels to ground. Lean back so your core is engaged, and twist from side to side. But don't take my word for it, check out more detailed instructions by clicking the link above.