We went down there with zero plans and did whatever seemed fun at the moment.
Tulum is really cool. We also did a half day catamaran/snorkeling/booze cruise that was awesome. We saw a giant stingray.
Personally, I wouldn't rent a car. If you are staying at a resort, you can book the outings when you get there, including transportation. Plus you'll have a guide that knows the area and the sites.
There's a catamaran trip/snorkeling/booze cruise that departs from Cancun and goes around the isle of mujeres. I'd recommend that.