walking is good for all round the tummy and sit ups help to tone what is there, if the tummy muscles have been stretched too much then there is no point
Get a heavy hula hoop they are easier to use. My Ju Jitsu training is really good for my core muscles, do a lot of ground fighting drills and you need to roll around for ages from side to side and round and round, try a local class maybe?
Sit ups are a good exercise, but some people find it hurts their back. If your one of them then lay flat on your back and raise your legs, start easy 4 sets of 5 raises with 1 minute break inbetween. Squats are great for your legs (and bum) and neither require any special equipment.
tiggerblue I got a hula hoop as thought it would be so much fun rather than the usual boring exercises as I used to love hula hooping at school, got it home and I cant do it anymore :-(