A few great exercises for exactly what you're looking to do:
1) For ABs: On the Pull down cable (Most people use it for Tricep pulldowns). Hold the bar up by your head, almost to your forehead with your feet about 6-7 feet from the base of the machine (Almost like you're leaning into the machine to take it from behind). Hold the bar to your head and use your ab muscles to do standing crunches down almost to your waist. I do sets of 15 at 60lbs. After you do each set, turn to the side with your feet exactly 90 degrees to where they just were and face the machine so that your side is twisted. Repeat the set for your obliques and switch to do the other side. Take a couple minutes to relax and do 3 sets total. You'll probably scream like a bitch and hurt real bad but that exercise is a beast.
2) For ABs/lower chest: Lay down on the bench with a dumbell (Start with 30 lbs so you can get the motion down). Keep your arms outstretched with the dumbell in both hands and stretch your arms back over your head till you almost touch the floor behind you and (While keeping your back and head flat on the bench) extend over the weight over your head, past your chest, and down to your stomach. Repeat that 15-20 times and take a break. Do 3 sets of a weight that's comfortable for you.
(Arnold actually raves about this exercise in Pumping Iron and credits it with expanding his rib cage
3) ABs: This is best done with a medicine ball but you can use a 20-30lb weight as well. Sit on the flat bench with the weight in both hands. Keep your feet outstretched so that you're basically balancing on your gluts (Ass bones so that your body is in a V). Move the weight from side to side, left to right, back and forth while maintaining your center of gravity and balance. Try to go as far to each side as you can while maintaining your balance. You should get a nice rip on your obliques and sides of your abs and you'll probably work your back a little bit too.
4) ABs: Another great exercise for Abs that is real quick, simple and easy but gets the job done is to get an Ab Wheel. I use that when I'm at home watching ballgames or whatever.
Here's the link to it:
http://store.yahoo.com/sportsdirection/ ... wheel.html
That thing will make you burn pretty good.
I know plenty more things for you to do but that should get you started.