The rent a car option is good - IF you know the area, are confident of driving under very different conditions than you are used to and you understand you MUST have insurance in most places (e.g. Mexico legally requires insurance for liability) and without insurance, you are quite likely to be detained until the issue is "straightened out" - can be days, weeks.

As well, what we call "local knowledge" is also on occasions invaluable, I recall driving into a petrol / gasoline station in Belize City only to recognize some very serious gang members and carefully drive on. Many places that seem idyllic have a poison pill possibility - areas you should absolutely not visit, rampant pilferage from cars (Bonaire comes to mind), traffic jam prone places, military checkpoints, etc. A rental agency is not going to spend much time educating you, a local driver or guide will know all about these things.

The Belize Zoo is truly spectacular, but out of town - natural habitats and local critters you are not likely to see most places. There are archaeological sites, etc.

Be aware activities are entirely at your own risk - there are no USOSHA or other regulatory agencies in many countries that are inspecting and licensing any of the parasailing, zipline, etc. and though I do not want to be a party pooper, you will become your own expert and inspector here, and liability laws in most countries do not protect you in case of injury. This is probably applicable to shipboard contracted tours as well - the cruise line will generally not stand behind any tour supplier, it's all on you. (They always claim to have inspected, etc. and then issue a strong disclaimer...)

OTOH, hiring a driver for four to six hours can be cheaper than renting a car, and the driver takes the hit for accidents / incidents. There are also, as others have indicated, local tour providers that are reputable.