when regular incline sit ups get old, i like to do medicine ball toss situps. on the incline, come up with the MB, toss against the wall, catch on the rebound, lower back down. the more you keep it over your head the greater work your abs do. you can do with a partner also.if doing with a partner, alternate left hand ,right hand, with a twist at the waist to hit the obliques good.
anlother good core exercise is the plank- elbows and toes on the ground , hold your body like a plank (straight). after you can do this for a minute or two, raise left arm/right leg,alternate right arm left leg.good for all around core strength.