It's going to be tough if not impossible to do on a day trip, because the earliest you'll get to Belize City by water taxi (either Caye Caulker Water Taxi Association or San Pedro Express) is around 8:30 a.m., and the boats leave from Tower Hill bridge near Orange Walk Town by around 9:30, leaving you little time to get a rental and drive the 50 or so miles. Likewise, coming home the last boat back to San Pedro is at 4:30, and you're unlikely to get back to Belize City and turn in your car by that time.
Now, if you want to stay over in Orange Walk Town, at least one night and probably two (Hotel de la Fuente is terrific, at less than US$40 a night double) you could do it.
--Lan Sluder