We got lucky with the weather with sun almost all day long. The Lonely Planet guide suggests getting there early to avoid the rain but this time of year (summer) there is no predicting. If you go on a weekend it's best to go early to avoid the crowds - bus loads.
You can either go to the Volcano or the Church. If you hire a guide you hire him for 2 hours or 8 hours - volcano being the longest. You can walk or ride a horse to either one but it's faster by horse. We paid 120mx (about $11) per horse and the same for a guide (360 total) to go only to the church. The volcano is a long trip so be in good shape and if you haven't ridden a horse in some time - caution - you'll feel leg muscles you didn't know you had for the next few days - even on the short trip.
When you arrive in Angahuan - on the road to Los Reyes - you'll be met by guides right on the highway. One or some of them are bound to follow you the mile thru town and out to the Las Cabanas outlook and the trail head. The guys that followed us are the ones we went with. There is also an old guide that usually leads large walking tours. The old guy might be more interesting with stories and geology. Our guide was just a young, nice kid that didn't talk much. They tell you they mostly speak Purepechan.