We (older couple) rented a small 4x4 at the airport, and had no problems. Driving is fine, but NOT into Guatemala. You should just arrange a day tour for that. I've heard that Caracol (we took tour) can be iffy for individual cars.
The ruins of Belize are many and great; but Tikal in Guatemala is certainly an amazing site.
I haven't used a travel agent since the 80s (the google is your friend); check out tripadvisor and
http://www.fodors.com/community/mexi...merica/belize/ for more detailed information.
By the way, it's only 70 miles from the airport to the Cayo area; the roads are fine, but watch out for speed bumps.