I voted for fighting monsters, overcoming obstacles, social encounters, and treasure. For treasure it's not straight up gold for XP(unless playing AD&D), but rather for acquiring something truly momentous, or managing to get the treasure out from under a monster without a fight. You might call that last part overcoming an obstacle, but since it's not set up to be an obstacle, I counted it under treasure.
I'd also like to add that as a DM I award XP for wondrous experiences, which isn't on the list. If you are a low level group and you manage to enter a fey land full of magic and mystique, I might award the PCs 500xp. If they actually met a god in person, they'd probably go up a level unless they were high level, and even then it would be a lot of XP.