What Exercises Should I Do to Work My Core?

OK--I have a question for all of you.  Please send responses because I really want an answer to this question.  What exercises can I do to strengthen my core?  I am currently doing the NuStep and the Rowing machine.  I hear the rowing machine is good for core work.  What else should I be doing?  Take into consideration that I am in a wheelchair.  I do have movement of all my limbs and can do many exercises with assistance.  So, just tell me whatever comes to mind and I'll worry about whether or not I can make adaptations to make them accessible.  I've heard Pilates can be good core work.  Do you think they are adaptable?  Should I even bother trying?