You should look into some ki aikido exercises. Ki tests as we like to call them, though it's not really a test...
There's an ok description here:
http://bodymindandmodem.com/CoolKi/Unlift.html (though it's an exercise for unification of mind and body rather than a 'cool ki trick to really freak people out......')
If you've ever tried to lift one of your mates when they're drunk and don't want to be lifted, it's a similar idea...
Unbendable arm is fun too, and you can also have a line of people trying to push you over, slightly different exercise, but same principles...
If the ki stuff's all a bit out there, try
http://www.aikidorepublic.com/aikiphysi ... sable-body
Feel free to drop me a PM if you want more information or a better explanation...