Ok, I like holding a medicine ball up in the air with my legs straight up and doing crunches to try and touch my toes with the ball.
Or, lie flat on your back and slowly lower both legs (or one to begin with, or keep knees bent) to just above the floor and then bring them back up again. The most important thing with that though is that you keep your lower back flat on the floor. If it rises up then take a lower option.
Or, lie on your back with your legs in the air and scissor them (accorss each other). Lower option is to keep them higher, advanced is to lower them, keep them straight and raise your shoulders off the floor.

For the obliques (sides) I like putting both knees over on one side with your torso square to the front and doing crunches like that.
Or, the "bicycle", bring the opposit elbow to knee and straighten the other leg off the floor (if you can).
Or, sit up on your bum and twist witha medicine ball so it ALMOST touches the floor either side of your bum. Advanced option is to lift your feet off the floor.

Or you can go the ggod old plank (hover). Go on your elbows and toes (advanced) or knees for beginners and don't move! Remember to keep dead straight and don't lift your bum in the air. Switch on your abs to protect your lower back and if you feel it hurting drop down a level (to your knees) You can also do it to the side - lean on your elbow and lift your hips off the floor. The lower option is to keep your knees and lower leg on the floor and the higher option is to push off so only your ankle and elbow are on the ground.

Phew! I've got heaps more but i'm too tired!