Petra and I met in La Paz, the world's highest capital (about 4000 m). There's not much to say about La Paz apart from the fact that you should be prepared for altitude sickness and you should leave it out of your tour planning because it's very dirty and polluted and there's not much to see there.
From La Paz we did a day trip to Tiwanaku, which is a very interesting site where the pre-Incan Tiwanaku population has resided from 1500 BC to about 1200 AC. It's very impressive to see how they developed their architecture and arts over the centuries. After 1200 they shall emerge into the Inca culture, which - despite the fact that it resides only 10% of the time of Tiwanaku - is world renowned, whereas few people have every heard of Tiwanaku.