Anything that I'm enjoying will easily make me lose track of time. I could be lost in a good book. I could be deep in conversation with a friend. I could even be playing Farmville or something equally mindless and time consuming. The only times that I'm really not in danger of losing track of time is if I'm doing something I don't want to do and I keep checking the time every three minutes.
Disclaimer: I don't actually play Farmville. I'm more of a SimCity kind of guy.