The *best* way, as far as enjoyable, fun, creative, would be to sail with Raggamuffin from Caye Caulker, island hopping and snorkeling and fishing for 3 days. At US$300 it's not the cheapest or fastest way though.
From Belize City rent a car and drive to Placencia, it is a beautiful trip and well worth the cost of a rental car. Besides once in Placencia a car is a nice thing to have as locations to places are a little more spread out than AC. A trip by car is also MUCH quicker than by bus.