Yes the humidity is an issue, in the jungle you'll find that your hands become soaking wet in a matter of minutes.
I used anti-malarial tablets (Avloclor) after seeking advice from my doctor. The advice was backed up from a family I stayed with in Belize City, they said why take the risk of not protecting yourself. I seem very prone to mosquito bites and despite arming myself with a potent mosquito spray I was still regularly bitten.
I had my own car so I didn't go on any tours. Although I didn't see any of Belize's most famous mammals in the wild, such as the Jaguar and Tapir. I viewed them at Belize Zoo, which is well worth a visit. You'd have a better chance of viewing them in the wild, if you hire a guide at one of the reserves. They may do night tours for the big cats. I believe the Tapir national reserve is closed to the public, although ATM tours pass through the reserve.
One thing is for sure, you'll see lots of vibrant-coloured birds, lots of Iguanas and countless leaf-cutter ants!
I really liked San Ignacio, it had a laid-back vibe. The mix of different people and cultures made for an eclectic choice of food too. If you happen to be in San Ignacio on the weekend, visit the market in the early morning, you'll see Mayan, Rastafarian, El Salvadorian and Mennonite people all trading next to one another.
Once I get some shots scanned, I will post them in this thread.