Some people spend their entire vacations (or at least the inland portion of them) in one of the adventure lodges of the region. The city of Belmopan [1] certainly doesn’t require much of your time, but the rest of the region may warrant a day or two.
As you drive the Western Highway [2] out of Belize City, save at least an hour for the Belize Zoo [3], and another half hour for the hike in Guanacaste National Park [4].
If you have the time, it is highly recommended to spend at least a full day visiting the other national parks along the Hummingbird Highway [5], and you might even choose to spend a rustic evening in one of the towns you’ll be driving through as well.
Links:
[1] http://www.moon.com/destinations/belize/central-belize/belmopan
[2] http://www.moon.com/destinations/belize/central-belize/along-the-western-highway
[3] http://www.moon.com/destinations/belize/central-belize/along-the-western-highway/the-belize-zoo
[4] http://www.moon.com/destinations/belize/central-belize/along-the-western-highway/guanacaste-national-park
[5] http://www.moon.com/destinations/belize/central-belize/along-the-hummingbird-highway