My sister and I went tubing on the river Mopan---it for the most part looks pretty safe---I think I had heard references that it was an easy float. NOT AT ALL--my sister is pretty athletic and she said she thought it was tough. I lost my float three times and had to dodge wacking my head (and certain serious injury--fyi, its my understanding there are no hospitals at all in Belize) on a huge tree.
Warning, it looked VERY CONFUSING to try to arrange a trip from Belize to Tikal on your own---its pretty far from the Belizean border (2 hours) and was basically in the middle of no where. The border crossing looked a bit complex---and I have made several land crossings myself (Morocco/Argentina & Uruguay/ Brazil & Argentina) and this by far looked overly involved. Also, there are tons of incidents of tourists getting mugged on the way to Tikal from Belize.
There are a lot of blood-sucking wood ticks in Belize. I found two on my head after a visit to Caracol ruins, so watch out if you're in the jungle. I woke up with one on my eyelid one morning - very unpleasant!
Not a very big deal...but...One afternoon/night in San Ignacio, the power went out for 5-6 hours.
The town didn't miss a beat as they must be used to this. Their power comes from Mexico, who's grid isn't quite what you get in the US.
We had dinner at Martha's during the blackout. They cook w/ gas and everything on the menu was available.
Kinda dark wandering the streets w/ all the lights out. We didn't wander far.