Sometimes the best stuff lies under your feet.
I'm not talking about underground sights, though some of those are fine - the Catacombs of Rome, the Moscow Metro, the Etruscan wells and shafts of Orvieto.
I'm talking about boring street engineering - the manhole covers, the tiles.
For instance in the Passeig de Gracia, Barcelona, all the paving tiles are copied from designs by Gaudi, with spirals and sea shells.
And in Reims, the town hall ordered special gratings with a fine wavy pattern, which have been used all over the city.
I wouldn't go a hundred miles to see them - but these little things often make me smile. And they do contribute to the feel of a place. And sometimes, as with the Gaudi design, they tell you something about the history of the place, too.
And some people take them much more seriously than me. Dan Heller takes pictures of manhole covers. Nice photos. Nice manhole covers. Worth a look.