If it's one of those flights that give you a copy of SkyMall, just flip through it and imagine how your life would be if you actually bought one of the items in it. Like those shoes with spikes on them so you could aerate your lawn.
If you have a laptop, bring a shitload of your favorite DVD's.
Make up stories for the people around you.
Look out the window and pretend you're a giant looking down on this puny world.
Make (real or fake) farting noises with a straight face.
Cut out the inside of your pocket so you could masturbate discreetly. (Thank-you Chuck Palahniuk!)
As for what you can do then i think the others above me have provided enough suggestions to keep you amused.
Oh and a tip for you. If you can manage to, try not to eat on the flight. Rather have something to eat before you get on and as soon as possible after you get off instead, sounds a little crazy but what it does is trick your body into thinking it's morning time and helps to re-set your body clock so you get less jet lag....less jet lag = more time to party on holiday )
Randomly stand up mid flight and loudly state "Excuse me this is my stop", then make your way to the door and see what happens.
Tell the pilot to do a barrel roll.
Ask people where there going (Cookie for reference )
Go to the bathroom every 10 minutes and change clothes.
Order as much peanuts as you can.
Make airplane references as much as you can.
Jump up and shout "I HAVE HAD IT! WITH THESE MOTHAFUCKIN SNAKES! ON THIS MOTHAFUCKIN PLANE!"
And please... enjoy your flight.