You can try the following:

1 ) Try to to kill Maghteridon, Doomwalker and Lord Kazzak
2 ) Go explore the old raids instances. BWL and MC will be gone or reworked after expansion. You may want to see that great raid instance as it is now.
3 ) Go and make AQ20 and AQ40 runs. If you manage to kill the first boss and twins boss in AQ40 the first time without wiping you will have my respect for life.
4 ) If you have a warlock in your team then go to ZG and enslave one of the demon imps you can see there. Even being a lvl 60 enslaveable demons, they are the biggest DPS enslaveable demon in the game. You will love to blow things with these. Hint, the demon fireball has 0.5 GCD.
5 ) Roll an unusual team just for the lols. I rolled 5 warlocks and they were a squad of fun and lols from start to end. Also have 5 mages and 5 priests teams in progress. Once you reach lvl 80 with these teams you can just use a toon or two of them to spice your usual group once expansion is released.
6 ) Go try "solo" Naxx. Lots of the bosses are quite doable and relatively fun. Expect some wipes until you get the right tactics.
7 ) Kite the dragon in Feralas to Ogrimar.
8 ) Kite the dragon in Duskwood to Stormwind.

9 ) Use an assist focus setup. Drive your team to the other faction town gates. Make a lvl 1 toon on the other faction side and make your other toons /focus on that lvl 1 toon. Then start marking people for death. Something like "XXX has been marked for death!" ... your other toons oneshot XXX... Everyone around laughs at XXX and he freaks big time. Just make sure you don't abuse that because most people don't mind being killed at the town gate once, but if you kill the same person 5 times in a row it's quite annoying for them. Be creative and have fun.
10 ) Same as before, but now fight 5 duels at the same time... it's doable but very difficult to with the 5. So far I have managed to win 4 duels from 5.