I have been trying to do situps but for some reason after about doing only 10 or 15 of them I start to have back and side spasms, cramps. However, when I take the exercise ball and lay on my back and place it between my feet and attempt to touch it I can do crunches that way with no problem so far. I did 4 sets of 10 reps and they are the most uncomfortable exercise to do.
I can't seem to get the sit-up thing either. I do all kinds of other things for abs but not those. I just can't seem to do them without back discomfort. I am eternally grateful that there are options for that!
Crunches on the Ball
Unless you live under a rock, you probably know that using an exercise ball provides for a very challenging ab workout. Instead of doing crunches on the floor, lie on an exercise ball. You'll challenge your abs in a whole new way while also working other muscles in your body like your legs and butt. Make sure you don't pull on your neck and roll forward a bit if the move is too difficult.
More: Ball Crunch
There are also some great suggestions, as well as a very fun ab exercise called the barbell rollout here. You'll need a barbell for it (or something heavy that'll roll in a similar way), but it's ridiculously entertaining!
I recommend checking out the rest of the site as well!
I haven't gotten my ball back out but you can also put your stomach ON the ball and wrap yourself around the ball, then do mini crunches that way. Your knees stay on the floor and it is relaxing to the back.
Crunches Suck!
that barbell rollout !! BUT you must be ready to be sore, sore, sore the next day 
