I spent 5 days in Caye Caulker just chilling, so if you want to just lounge its a nice place to do that. Do not stay in Belize City, it is awful. Do the ATM tour in San Ignacio, the price was way worth it. If 3.5 is to much just take a bus to Tikal and Flores in Guatemala, its not that far away.

I think 3.5 weeks is reasonable. I have spent a week on Caye Caulker on 2 different trips. You can dive/snorkel or just chill. Raggamuffin does a fun 4 day sailing trip from Caye Caulker to Placencia. Nice uncrowded ruins near Placencia (forgot the name - we just took the local buses to get there). Dangriga is a cool place if you want to experience the Garifuna culture and they have a music festival there. If you have extra time, Tikal in Guatemala is definitely worth a visit.

Caye Caulker is lovely! We rented a canoe and paddled around the island, very nice! We went on a cheap and funny snorkeling trip with a boat (raggamuffin?) Just go to the Split, and look for the most colourful boat :) Swimming with stingrays was amazing! Snorkeling was good, clear water and lots of fish! I loved it! Don't go to Hopkins, I read about it in LP, but it is the only place in CA i didn't like. Very dirty and narrow beach, dirty water, weird atmosphere and no good reataurants.

I had only one week an much ideas fo Belize, The first day I came to caye caulker and I stay there for the hole time....

3.5 weeks in Belize is a long time, but to visit all those places it's a good amount to enjoy it without feeling rushed. San Ignacio is fun with plenty of activities to keep you busy for a few days. The ATM tour is great as already mentioned. If Cayte Caulker is too low key and quiet for you (do the Raggamufin snorkel tour) then Ambergris Caye..more developed, is within spitting distance and you can do day trips between the 2 islands. Definitely hit up some parks and, I agree that a visit to Guatemala is a must if you haven't hit Tikal before. Placenia is a good spot to base yourself if you want ot go kayaking or explore some uninhabited islands offthe coast too.

it might be too much. Caye Caulker is too small to spend more than three days. Diving is very expensive. There is not much fish in the home reef and you would have to pay a bit more to go to the Hol Chan Marine Reserve, where there is also not a lot of fish but a lot of human-fed nurse sharks that get nervous.