Hammer Swings!
Yes, simple hammer swings. All you need for these is a tire and a sledgehammer. You simply use your abs to drive the hammer down onto the tire and stop it`s momentum when it bounces back up. This kind of training is all power and you will quickly feel exhausted while doing them.
Now there are a couple of variations you can do:
One is the angled chop. Simply swing the hammer at an angle so that you focus on one particular side of your core. Be careful with these as the bounce of the hammer can be unpredictable!
You can of course do overhead chops. The trick with these is to keep your back straight. As you get tired you will start to let your spine curl up which can lead to some very nasty injuries! Always keep your back straight!
If you have the strength for it, you can do one handed swings. These can be very tricky and the shock of impact can be enough that your hand goes numb within the first couple swings. These are great for training your sides but you should not do them too often to avoid hand/forearm injuries.
The final one is the horizontal swing. You have to rig the tire so that it stands upright at chest height(I use rope). Dig your feet in and swing straight to the side. This can be the hardest variation to for keeping control of the sledgehammer so make sure nothing you don't want smashed is in the near vicinity!
These are a favourite for training for six pack abs and I am sure you will like them too once you give them a try. If you cant find a tire(I got mine from the dump, it is illegal in some places!), you can use sandbags or an axe/wood combo if appropriate. Remember that you can build six pack abs through this kind of training but to really show them off you will need to lose fat until we reach a relatively low bodyfat percentage.