We went to Placencia, Belize a couple of years ago. It's much smaller than Belize City, rustic and newer at the same time. We were there for 4 full days and 5 nights. You can walk almost anywhere in town. We snorkeled Laughingbird Key (Google it) about 11 miles off shore, we did a taxi/tour of the Mayan pyramids and swam in a blue hole on the ride home (5-6 hours in the car) and we did the Monkey River Jungle trip.
If you are expecting 4 and 5 star facilities you will be disappointed... this is a more rustic and just developing area. Now we did rent a big house (5 bedrooms, 2 kitchens for 8 people) that was directly on the beach and had a nice pool between the house and the beach. The house was very 5 star with wonderful facilities.
We had a great time and I'd go back in a heartbeat when the chance to do so comes along. If you have questions or want to see pics, just let me know. Also happy to go to email and keep it more private if you want.