Potosí

this lovely city was required by law (during colonial times) to have all of its buildings painted in different colours, the result is incredible (even more so in contrast with Sucre, where everything is white, or even La Paz, particularly El Alto where walls are left unpainted in order to cash in on tax break for ´unfinished´ construction). furthermore, being an incredibly significant concentration of wealth a while back (Cerra Rico alone pretty much bankrolled the spanish armada, et cetera) the streets have an incredible history of extreme wealth. be warned Potosi is almost always extremely cold.
last but not least, be sure not to miss out on the chance to go on a tour of (in) an active silver mine.