We just returned two weeks ago from a two week vacation during the "off season" in Belize. The first three nights were spent on Caye Caulker.
We loved the laid back "Go Slow" vibe and especially enjoyed the free bikes and kayaks that came with our Leeside Room accomodations, which was situated on a quite back street just past the soccer field. Leeside was great for few reasons: air-conditioned!, comfy queen bed, hot and cold shower. We only wished there would have been a CD player or stereo instead of the cable tv, which really wasn't necessary.
Definately eat at Rose's next to Habaneros. You pick your fresh fish from the barbeque grill on the street (we had grouper and snapper, but there was also plenty of lobster too). You get two sides and an appetizer for $7.50 to $12.50 U.S.
We recommend Amor d' Cafe for fantastic homemade bread, yogurt, granola and smoothies. Bars all over town have happy hour where you can get really cheap rum drinks and Belikin beer, which is fantastic.
We also ate at The Jolly Roger, which we had heard about, and although the food wasn't as good as Rose's it was amusing to watch the big guy work the crowds and hold court as people walked by.
Just touring the island on bike was a favorite for us, along with kayaking to "the other" Caye Caulker
on the other side of The Split.
If you go on a snorkel tour, definately go with Carlos Tours. We had 6 people in our group to Hol Chan, Shark Ray Alley, and Coral Gardens. Carlos is very knowledgeable about the wildlife and eco conscience. He takes amazing underwater shots of your tour and you can purchase a disc of them afterwards for $15. They're some of the best photos we have.
Sunrises and sunsets were another bonus. We had perfect, but hot, weather and were happy to not have to deal with crowds or a lot of other tourists.
Note: because of the mangroves the sand can smell pretty strongly of sulphur - it's not your partner passing wind!