Aaron Marino of alpha m. demonstrates crunches on the bench, which you can do pretty much anywhere. This is one of Aaron’s all time favorite ab exercise.
Sit on the bench with hands behind, and then rock back so feet are off the ground. Lean back with knees in and crunch. To make the exercise tougher, straighten your legs out. Even more intense, add a dumbbell to your feet for more resistance. Aaron also demonstrates this exercise on the floor which doesn’t have as much range of motion but you can still keep the tension on the abs.