i got invited to sit in the captain's chair for a photo on the 747 once. As long as the plane is blocked in at the gate, I don't think there's anything that you can do to mess up the plane. I remember seeing some cables attached to the plane when they're blocked in, I'm guessing it disables all inputs, so if you hit the throttles for some reason, nothing will happen.
I really wish they had a flight deck camera that they could play on the IFE screens during takeoff and landing, that would be really cool!