What was the worst and the best moment in the classroom for you?

1) Is there a good exercise I can do that only utilizes the body? I've no money for the gym and all I'm currently doing is some stretching, squads, push ups and crunches in the morning. Is there more to do?

2) Is there a way to make push up training more effective? I feel as though I'm plateauing at 20 right now. Is there a way to get gains more quickly?