For me, "for fun" headphones will emphasize convenience, appearance, comfort, and uniqueness in their appeal, while still having sound quality that is at least acceptable.
My favorite "for funs" are:
- Parrot Zik - bluetooth noise cancelling over ear: you have to be intrigued by their stark, Starck (as in Philippe Starck, designer) appearance, their iPod app that changes the virtual sound stage and equalization, their no-button touch controls for volume and song skipping;
- HeartBeats IEMs -- good bass without messing up the treble, most easy-to-insert IEMs I've had to get a good seal, flat cord for convenience;
- Sennheiser PXC 310 BT - almost as small as IEMs, these wireless folding noise cancelling on-ears will actually fold to fit into your shirt pocket.
What are yours, and why?