We took the Moon Valley tour, actually only because we had a day left in La Paz and didn't know what to do. The tour was ok, not really something special. You have just one stop, at the Moon Valley - that's why they call it the Moon Valley tour I guess ;-) - and especially if you are still going to (or have been to) Uyuni, the Moon Valley is not really something special.
Also it's not really a hop-on-hop-off bus, as there is only one stop and the bus only does each circuit two times a day. So if you take the Moon Valley tour in the morning, you can stay at the Moon Valley (hop off) and get on the next bus about 3 or 4 hours later (I would not recommend this, as you can see the Moon Valley in about 20 minutes).
The website says the ticket for 60 Bolivianos includes both circuits, but after the tour, the hostess asked if we also wanted to do the Downtown tour for another 60 bs.
One thing, in case you are over 180 centimeter (about 5 feet 10), it's not really safe to sit on top of the deck. I was hit by some electricity cables and almost hit my head against a tunnel! I suggest the should use warning signs for this.
All by all, no need to do this tour unless you are really bored (and even in that case, I think I would prefer going to the witches market for the third time instead of taking this tour).
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