You're much better off going on a guided tour if you'll be accommodated on AC. You can do it yourself but you'll miss out on so much if you do it self-guided. As an example one of the best parts of the Lamanai tour is the run up the New River where you'll see all kinds of flora and fauna. You'll miss all that if you drive in and the drive itself can be very rough depending on the time of year. By road Lamanai is 100 miles round trip from Belize City, by the time you fly to Municipal, rent a vehicle, pay the outrageous fuel costs, find somewhere to eat and pay the park fee you'll probably spend more money doing it yourself.
Your other option is to spend 3 nights in Cayo as it's centrally located to many activities, including hiking in Cockscomb Basin Jaguar Preserve.