I was in Belize and Peten, Guatemala in early 2007 for some bicycling. What I would recommend in Belize is:
Swim in Blue Hole and swim into the cave (strap a miner's light on your head and make funny whale sounds as you swim in :-).
Check out the ex-pat artists in Placencia.
Snorkel the barrier reef out of Caye Caulker or Hopkins. If in Hopkins you can stay at Jungle Jeanie's. She's an expat canuck and has some neat little cabins. Or camping on the beach, as I did.
If you're into birds then check out Cockscomb Basin. There's an 11km dirt road leading to it and there's camping or very rustic cabins.
The Belize Zoo is a must. They're all rescued animals and the entry fee supports a good cause.
When I was in Cayo I rented a canoe and paddled upstream for 11km's. Then lazily drifted back while watching iguanas, tiny bats and birds as dusk approached.
My experience with Guatemala was somewhat limited. But Tikal is a must. Even though it's overrun by turistus autobus obnoxius, it can be simply incredible after they all hop on their fume belching monsters and head home in the afternoon. There's camping right at the site and it's well worth staying overnight (hotels are available as well) so you can enjoy the quiter times.
While I was in Tikal I also cycled the 50km return trip up the dirt road to Uaxactun for something a bit quieter. Absolutely no tourists, just a little sleepy town of 300 people and some smaller ruins. Very peaceful.
If you can, take the boat trip up and down the Rio Dulce from the town of Rio Dulce to Livingston and back. It's quite beautiful.
You can check out my sometimes irreverent blog at
The Restless Biker. It's mostly bicycling but it could give you some ideas.
Michelle