Mixco Viejo is around 60km from Guatemala, but the road is very slow and you can expect to take around 2h30 to get there, by car. Take Calzada San Juan, and pass through San Pedro Sacatepéquez and San Juan Sacatepéquez. The road is straight forward, but signalization is almost non-existent.
There's an admission fee of 5Q for Guatemalans and 50Q for foreigners.
The ruins close at 4pm.
Walking around the whole park should take no more than 2 hours.
There are many well preserved ruins, including a court for the traditional Maya ball game and many pyramids. Compared to Tikal they are very short, but are still impressive.
If you are lucky, you might witness Mayas performing rituals.
There is no store in the ruins.
Bring water, as there is nowhere to buy it in the ruins.
There are no lodging facilities inside the ruins.
You have to come back the same way you came. On the way back to Guatemala, it's possible to get a chicken bus to Cobán. If you are not by car, you can get pick-ups or chicken buses.