There's just enough left of the Tiwanaku site that I was able to get a decent appreciation for this ancient culture. The museums are very well done, featuring several impressive stone sculptures.  The highlight of the museums is the Bennett Monolith, an amazing 7-meter tall statue in excellent condition after over 2000 years.  It survived the Spanish by being buried until rediscovered in the 1930s. Out in the main site, we started our tour at the Akapana Pyramid which gives a nice overview of the site.  While the Sun Gate on the Kalasasaya courtyard is probably the most famous artifact here, I really liked the semi-subterranean temple.  This sunken area is where the Bennett Monolith was found. The walls of the temple feature rows of numerous skull sculptures, some more like a human skull and others are more stylized.

The auxilliary site of Puma Punku is also worth a short visit.  There isn't a lot to see there, but it is worth checking out the jumble of remaining building stones and walking around the perimeter.

I joined an organized tour from La Paz that was pretty cheap (about $US 13), and had a very good guide who did a nice job of explaining the site. Overall, I enjoyed learning about another impressive pre-Columbian culture.