(In no particular order)
- Sing the Spongebob Squarepants theme song in the captain's voice
- Sing the opening line to "You're Beautiful" in a James Blunt-ish rasp
- Say things in an exaggerated French accent
- Say "That's weird" with a British accent
- Say things in an effeminate British accent
- Sing "berries and cream, berries and cream, I'm a little lad who loves berries and cream"
- dance while singing "berries and cream, berries and cream, I'm a little lad who loves berries and cream"
- imitate Heath Ledger's portrayal of the Joker
- Say things in Japanese
- Say thank you with a Japanese accent (senkyuu)
- Use a Scottish accent
- Say "They can take our land, but they can't take our dignity" in a poor imitation of Sir William Wallace from Braveheart
- Rap (idthink this is always forbidden though)
- Imitate Nikki Minaj
- (addition 03/06/2011) Sing anime song from that one episode of South Park, "Hey! Hey! Let's go! Kinkasu"
I don't remember everything I'm disallowed from doing, but if I remember a significant amount more, I'm going to post an additional entry.