Do you have access to dumbbells or a gym?
Without them, the things that come to mind besides push-ups and sit-ups are lunges, squats, tricep dips on a bench or sturdy chair, and calf raises done on a stair step. Actually, burpees, mountain climbers, plank, and boat pose come to mind too.
When you do sit-ups, do you do four variations (regular, left oblique, right oblique, leg raises)?
The key to building muscle is to do your exercise until the muscles can't perform any more (known as "to fatigue"), rest, and then do it again (your fatigue should come much sooner), rest, and do it again. Give your muscles plenty of lean protein and carbs (look up "recovery meal" to get an idea of what I mean) afterwards, to use in the rebuilding process, and then rest those particular muscles for at least two days while you do other things.
Hopefully anyone who disagrees with me will post here and help you with different advice