Driving to the ruins is real easy, I wouldn't let nervousness about driving keep you from going. In my opinion Coba is definitely worth the trip, Tulum not so much.
True, rent a car in Playa and drive down.
Tulum is a post late-classic period site, the buildings and temples are not as big and impressive as those of Coba or Chichen Itza, but the history and architecture is very interesting. The way the buildings line up with the stars and sun and moon is an amazing feat!
Coba is really just getting excavated, although it's been going on for several decades, the 100 square km city has only "the tip of the iceburg" discovered if you will, but it does have the tallest temple in the Yucatan, that alone is worth the drive.
With a little planning, you can make both sites in one day. Get an early start and go to Coba first to beat the heat, it can be very opressive once you get into the archeological site, because you are litteraly in the middle of the jungle and this site has not been excuvated a lot compared to places like Chichen Izta (which can be opressive on it's own), and Tulum, so the trees are very thick along the paths, (which are the width of a road, not narrow at all), but still the air can just hang with heat because very little breeze makes it through the brush.
Once you are at the site, you can hire a private guide to take you around. We did this several years ago, with tip it was something like $30, could be more now, but it is worth it, you will get sooo much more out of the visit. Especially Coba.
The highway from Playa to Tulum is practically brand new and a very nice drive. From Tulum to Coba is a road more or less like our "farm to market" roads here in the States, (2 lanes, paved.)
There are 3 small Mayan villages along the road to Coba, in the second one there is a small Mayan hut along the road side where you will find a man that makes Mayan hieroglyphs out of limestone blocks and other Mayan sculptures. Stop in and say hi to my friend and get yourself a very cool and unique suvenier.
You can identify his "shop" by the scupltures outside of it, he used to have a 6' stelle (plaque monument) planted outside, it's on the right hand side of the road when you are headed to Coba.
Tulum is great because it sits right on the ocean and once you are inside the city walls, the ocean breeze helps keep you cool.
Something I didn't see mentioned was the swim with stingrays and dolphins.
...probably a reason for that, neither are very popular venues with this crowd
ON ISLAND:
1. Second vote for breakfast or lunch at the Museum...a must. Then go down to the first floor in the back behind the grand staircase is a man in a Mayan hut that will give you a run down on the day-to-day life of the Maya...leave my friend a nice tip please
2. Definatly drive around the island and explore. El Cedral (100 year old town in central Coz.); go see "Tree" the HUGE Cieba tree just shy of Punta Sur; Punta Sur Park...climb the lighthouse! see the gators!; Eat at The Feedom Bar (Bob Marley's), whole fish at Chien Rio, sample the awsome margaritas and fish burgers (yes, fish burgers) at Coconut's, stop at some of the road side shops for cheap souvies; find a secluded beach and laz the afternoon away in the Carbbean breeze, often the breeze comming off the ocean on the "wild side" of the island can feel like God's air conditioning on a hot day.
3. San Gervasio if you don't make it to the Yucatan for a Mayan adventure, this site's buildings are more on the line of the temples in Tulum, (smaller and less grand) rather than those of Coba or Chichen Itza, but it is much larger a site than it appears and good place to kick off Mayan interests.
