I noticed that when exercise is done in groups of 100, broken down any reasonable way you want:

1 set of 100 jumping jacks

2 sets of 50 sit-ups

5 sets of 20 push-ups

10 sets of 10 pull ups

etc...



with or without weights works wonders.

even air punching (shadow boxing), jabs, hooks, upper cuts, etc... (broken up into reasonable sets).

in 10 to 15 mins you start to feel tight, in about 30 mins you are sweating, you could stop there or go for an intense workout, 100 arm exercises, 100 leg exercises, 100 neck exercises, 100 stomach exercises, etc...)

it's fast, simple and effective