Tiwanaku is a very nice site just an hour and a half from La Paz. It has a very interesting pyramid (though the botched repairs nearly cost it it's UNESCO status).
The Puerta del Sol is the star attraction, but much smaller than you would imagine.
The museums are a bit poor, but the giant statue is very impressive. You get a ticket that allows you into the two museums, the main site and the Puma Punku site for 80 Bolivianos, so it is actually a very good deal.
I would recommend that anyone interested in history or archaeology visit. We booked a tour through Julius travel and we paid 180 Bolivianos for transport, entry and lunch.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.