There are lots of variations of these. One of our favourites and most effective is the single leg crunch.
Lie down on your back on the rebounder with your neck straight (use the rolled up towel under the neck for support). Keep one leg on the ground and raise one leg up with a very slight bend in the knee. Run the hand on the same side as your raised leg up the side of your shin whilst the other hand is by the side of your head.
Focus on moving the bottom of your ribs towards your hips without pushing the chin forward. Exhale on the way up so your belly moves in.
Repeat with the other leg raised.
Your abdominals should be screaming after 1 set of 20 reps!