Anything beautiful. People, places, flowers, ants, toenails, whatever you think is beautiful. It's not really that complicated.
The only restrictions are
1. No harsh shadows caused by a flash (built in flashes are the biggest problem here)
2. No redeye. Its just not right.
3. Portraits of pretty girls are okay, but if you just take pictures of girls asses while they walk down the street, i don't know, but something about that doesn't scream "art" to me, so let's not add them please.