Do two different kinds of sit ups to work all the stomach muscles.
The normal kind.
And like this: lie on the floor, face up, so that there is something behind your head you can hold onto with your hands (such as your bed). Then keep you legs straight, point your toes and lift your legs off the ground. Raise to about 20 - 30 degrees then lower to being just off the floor, and repeat.
WARNING-do not try and do too much of that at first if you haven't worked the lower stomach muscles before, as they are hard. I pulled all the muscles in my lower abs by doing too much too soon - start with five, if you can, then work up adding one a day. Seriously I am talking from personal experience. However, once you build up, they are excellent for toning that area.
Then of course there are press ups, and also find something heavy in the house you can hold (filled up water bottles, cricket bat, whatever) so you can do normal bicep curls as well.