What about exercises which don't imply weights, for example push-ups, squats or abdominal exercises, is it safe to go beyond 15 reps ?
You can accomplish alot simply by purchasing 1 weight plate, like say 25 lbs. Have someone put the plate on your back for your pushups, or you could do it yourself but its just a little more difficult...can be done with some manuervering though. For squats hold the plate overhead, if thats to easy do the squatting one leg at a time. You could also do wall squats with it, clutch it to your chest and sit against a wall like theres an imaginary chair there. Have your upper legs parrallel to the floor to start with, and hold that position for as long as you can. Trust me, it can be incredibly grueling towards the end! When you can do it for a minute, sit down to a lower position or do it with one leg at a time. Abdominals you do weighted crunches.