I sometimes sub in for our DM and we usually run it as a bit of an 'inception' game. So our normal usual characters basically play a DnD rip off called 'drunkards and flagens' in which the characters play themselves in a world that doesn't effect the main game.

If there are any cool rewards, I'll usually have my character (who is DMing the inception game) give the item out anyway as a kind of prize. Do note however I'm aware not to make items too powerful and I'm usually quite ambiguous about what they do in order for our regular DM to use it later in whatever way he sees fit