Cahal Pech is walking distance, albeit uphill, from San Ignacio. I paid US$10 for a guide, which took about an hour for the tour. This was my first Mayan ruin, and since the excavated portion is in a consolidated location, it was easy to traverse the space and get an explanation on what the different buildings, ball courts, etc were used for. I found this to be a good basis for touring Caracol and Xunantunich later on, especially as no guides were available on a walk up basis at Xunantunich.
The Museum is small and dark, but I spent about half an hour reading the posters after my tour (which I started around 8am on a Monday morning), before other tourists arrived.