First you need to check if your rental car agency in Cancun permits its vehicles to be taken into Belize. Most don't. A few do. If you drive into Belize you'll have to get Belize insurance, which is inexpensive and you can buy it at the border.
The only main road in northern Belize is the Northern Highway, which runs from the border about 85 miles to Belize City. It is a good paved two-lane road.
There is no road to Ambergris Caye, which is an island, though only barely separated from Mexico by a narrow channel.
There is a pedestrian ferry, actually two, from Corozal Town to San Pedro. You would have to leave your car in Corozal. There are safe places to park it, such as at Tony's or Corozal Bay Inn, but you would need to check with these places for permission.
Depending, you might just be better off going by bus to Chetumal/Corozal and taking the ferry from there, as you would not have to pay for your rental car for the time you were in Belize.
--Lan Sluder www.belizefirst.com