RE: Pushups/free squats
Sit ups should be easy enough
Press ups keep doing them till you cant do any more drop to your knees and keep going. when you cant do them stand up and do them against the wall this will REALLY burn but it's also good training for the muscles
As for the squats use a barbell (medium weight) squat till failure lessen the weight and keep going (no rest just change the weight and get back on it) work you way down to nothing. when you cant do any more have a rest and start over.
There is also a home made drink you can use that will help reduce the amount of muscle burn you will feel (ill find out what it is)