some of my 'must see's would be the zoo for sure. located between belize city and belmopan. maybe even spend your first night there, across the road at the tropical education center.
then take in a night tour of the zoo as well.
st. hermans cave, but cave tubing depends on water levels too.
inland blue hole is so-so. hike about 20 yds and see the hole. it's Ok,, but you won't spend much time there.
hiking to waterfalls is high on my list.
billy barqider (sp) is my favorite,, no body hardly ever there! sshhh,, don't tell anyone.
mama noots has some nice trails and waterfalls also.
and any trails at the jaguar preserve. they also have river tubing.
the ruins down south near PG are OK. no big temples but a nice drive and worth the time.
once in placencia i recommend a day trip snorkeling, followed by some local bar action.