I've been to Belize twice for the "surf and turf" trip described by a couple of people above... a few days on Caye Caulker followed by a few days in San Ignacio. On my second trip, I ended up road tripping to Tikal with a couple of people from the lodge where I was staying. (Parrot Nest. Fine, but not awesome. Would stay elsewhere next time.) We stayed at Casa Azul in Flores, which was lovely. We were there in October, and we didn't get to Tikal till late in the day, but it was fabulous because there was almost no one else there. Lots of animals out and about, some Mayans performing a private ceremony, etc. Really cool.
Keep in mind that $1 US is $2 BZ. I paid double for my first taxi because I didn't confirm which currency the driver was talking about. People will often give you a price and hope that you assume they're talking about USD. So ask! And take lots of small bills with you.
As noted above, Belize City is only good for flying in or out. I wouldn't stay there even one night. Get off the plane and go directly to a water taxi or a bus. It probably doesn't matter, but I went to Caye Caulker first, then inland. There isn't much on Caye Caulker, but it's very good for relaxing, enjoying some local food (and very watered down drinks, in my experience), etc. Might be a necessary period of recovery if you've been working hard pre-vacation. Then you'll have energy for the more active things to do inland. I stayed at Mara's Place, a very basic but cheap hotel right near the split. Haven't been back since they rebuilt the split. I&I Bar (Rastafarian) is worth the short walk from wherever you're staying. It's 3-4 floors high, mostly open porches, and most of the seats are swings or hammocks. Makes you feel like a kid again. Oh, and Caye Caulker Bakery... the cinnamon rolls...!!
A local "chicken bus" (a recycled American school bus, often with the American city or county still written on the side) will take you from Belize City to San Ignacio for, last time I was there a few years ago, $4 for the entire 3-hour trip. It's not luxurious, but it's darn cheap, and it's a neat peek at the local culture. In San Ignacio, I stayed at Midas, which has cute little cabins that are surprisingly cheap (about $50/night). It's a very quick walk into San Ignacio from there (maybe 10 minutes max). If you're into glamping, Chaa Creek, which is a very expensive resort, has a nice little area where they only charge $50 or so per night. I haven't stayed there, but it looks nice.
I can't seem to find it online, so maybe it's not there anymore, but in San Ignacio, I had the best Indian food I'd ever had in my life. I think I ate there 3 days in a row. It was just a patio behind an Indian couple's house with a little sign out on the sidewalk. She cooked, and he served. Service was super slow, even though they only had 3-4 tables, and their little boys were running around on tricycles, but it was still a pleasant outdoor dining experience, and the food was out of this world. Maybe ask around once you get there if you're interested.
ATM cave, as mentioned above, is definitely worth doing. Also the local Mayan ruins, and if you can get to Tikal, definitely go for it.
Oh, I almost forgot to add that I agree with the recommendations for Raggamuffin Tours. They're a lot of fun, and reasonably priced! One of these days I'd like to go back and do one of their overnight trips.
