Do your homework before you go
Mixco Viejo is a beautiful Mayan site located about 1.5 hours outside of Guatemala City. The admission charged to foreigners is Q25 (or roughly $3.33) per person and the roads to get there are good, although there is no directional signage until you are fairly close to your destination. I recommend checking with a local for detailed driving directions before you leave.
I say do your homework because once you arrive there is a relief map of the site with the various plazas and areas labeled, but there is absolutely no other information (in any language) to inform you about the site or its culture anywhere on the property. A guide book I own referred to a planned museum, but according to the worker I spoke with this project was short lived and no longer exists.
Despite the lack of information, the views alone are worth the trip and the remains have been well restored. Some of the taller temples provide an excellent view of the site from their summits. Be sure to note the carved serpent heads on either side of the ball courts.
One caution: On the day we went there were some folks working in the parking lot. My wife was not feeling well and stayed in our van while the rest of our group went in. Not 5 minutes after we left, one of the workers (not realizing she was still there) tried all of the doors to see if they were locked. Definitely lock your vehicle and secure your belongings!