I'm not sure which to recommend more highly - the toilets or the tapas at Set de Gotic, a restaurant on Carrer Montsio. We found it in the old city after finishing our walking tour of Gaudi buildings. I don't think I've photographed toilets before but visiting them was quite a surprise. Incredibly ornate tiling throughout. I had to whip out the camera whilst in the loo.
If you didn't need to spend a penny, enjoy the tapas instead.