Costa Rica and Belize are amazing, I've been twice.
Couple things to see or areas to visit
1. Mount Arenal - amazing volcano and tourist area, lots of places to do river rafting, ziplining, etc
2. Ruins of Tikal - Just across the belizean border in guatemala is the ruins of Tikal, amazing Mayan ruins completely intact (They are the ruins you see in the Star Wars IV) But more importantly if you get a local guide it will provide you a ton of insight and historical context with regards to the mayan culture.
3. Sacrificial caverns in Belize. I can't remember the exact name of these caverns, but this is easily the top vacation moment i've had in my life. I had to hike a half hour through the jungle to a point where a stream/river exits a cave mouth and then hike up the stream (into the cave) about a kilometer....things get pitch black, and then you get to jump up about 10-15 feet into an underground area larger than a football field with a roof 100 feet high. Keep in mind the only light you have is a headlamp on your helmet. You then get to hike in another 100-200 metres to where the mayans used to sacrifice people to their gods. (caves are extremely holy places in the ancient mayan beliefs). You actually get to see the body of a princess that was sacrificed a thousand years ago and was preserved....it was incredible and I would highly recommend checking it out because words can't give it justice.
If you have some questions toss me a PM i've been to alot of different central american destinations
