1. Sit-ups, push-ups, sit-ups and leg lifts, prone stretches, squats, frog jumps, etc. Belong to the same category.
2. Running. Running is a very good exercise, and it requires little environment. A towel and a bottle of water will do.
Swimming. A good project to lose weight.
Aerobics is fun and healthy.
In fact, as long as you are careful, you can exercise well without equipment.