When it is a beautiful day around you but you are feeling a little down, and nothing in the world seems particularly great, but as you walk through the beauty unnoticing you see a pretty person smiling, and it is a real and true smile, and even though it might not be at you you are infected with their happiness, and suddenly everything that seemed a bit dull and boring and mundane becomes bright and cheerful and generally awesome, and all the crapness is forgotten for a moment and you just smile too. This is probably my favourite thing.
Actually, just smiles in general. When someone is smiling and they really mean it. That's pretty awesome.
Also, when you find your passport in the crisps.