Making plans to visit Belize in May 2012 for 5 days. We'd like to visit the Mayan ruins, visit caves, scuba, horseback riding and beach it. Where would be the best location to stay to do these things. I'm thinking San Ignacio?

Other questions:

Best ruins to visit?
Best caves? (thinking ATM)
Best beaches?
Recommended hotels/lodges?

We are also thinking of going over to Tikal, Guatemala if that's worth it?

Thanks for any input!

There are many options in the San Ignacio area; an excellent place to stay. From there it is easy to get a day tour to see Tikal (well worth doing, and you can see quite a bit in a day trip).
The Mayan ruins of Cahal Pech right in town are, IMO, a must see if you are staying in the area. Very beautiful, very do-able site (in a few hours or spend all day). There are many other ruins (more famous) that can be visited in the vicinity.
ATM cave absolutely, also a day trip tour from San Ignacio.

Beach-wise, Ambergris is a little disappointing; not really a swim or snorkel from shore place (hardpack ground, seagrass). But that's where the biggest choice of lodging is (from cheap to incredibly pricey).
There are other shoreline places where you can enter the water and snorkel, such as Glover's and Tunereffe, and more remote islands/atolls.