Originally Posted by stoneric

Sit-ups won't get you a six pack, they'll give you a bad back. Crunches, front and sides. Reverse crunches, front and side. Side bends with your dumbells. But with all of that you'll still won't see a six pack unless you get your body fat down to a certain %.

Push ups are great. They simulate the bench and can be done with enough variations that you can work out almost all of your upper body muscles. You can get a good burn on your biceps with your 20lb dumbells. Just do a shitload of reps with little rest between sets. Do the last couple sets to failure and your arms will be pumped. But again, you won't have a ripped look unless you pay strict attention to your diet.