Hi Phydeaux - I would respectfully push back on your statement that "tours are for people that can't." The decision to choose a tour operator or to travel on your own is very personal and dependent on the situation. We have been to RM numerous times and have done all of: organized tour, rent car and drive, take cabs. We have not taken a bus/collectivo and there are those that will say that if you are paying for a taxi you are a fool. You have to do what is best and most comfortable for your party.
1. A very large number of activities along the Riviera Maya are located on the one major North/South highway. You can literally drive south to Xel Ha, Xcaret, Xplor, Rio Secreto and turn off the highway into their respective parking lots. You can very easily get to Playa Del Carmen, Akumal, and Tulum with maybe two or three turns. The road, especially during the day, is quite safe and controlled by police/army. I felt very comfortable driving, heading into town, getting gas, stopping at grocery stores etc. If you are going far afield, like Chitchen Itza, there is an added degree of difficulty. Not at all a problem, but just a bit more complicated and off the beaten track.
2. Xcaret, Xplor and other parks will have buses that pick you up at the resorts and take you to their location. There is an upcharge in most instances for the transportation which may or may not offset the cost of the car rental. Here, it is basically deciding if you want the convenience of the bus picking you up at the resort and driving you home (after what might be a long day) in the evening. Or, do you want to rent the car, pick it up, drop it off, etc. Some resorts, like Vidanta, have you park in a lot that takes quite some time to get to and back from. If you go to Xcaret and stay for the evening show it is a very late night and if you have to weigh how tired those in your party will be.
3. Tour guides - depending on your location you may want/need a tour guide. Obviously, in some locations you can find a guide at the destinations. In others, they may need to be pre-arranged. In our tour through Sian Khan having a naturalist tour guide was fantastic. We have been on many nature/wildlife tours where an expert guide will make or break the trip.
4. Many cab drivers, guides, etc. work really hard for themselves and their families. I don't mind giving back where we can. I believe altournative might even give back a bit of the profit from each tour towards conservation efforts. Feel free to be skeptical if you want.
5. Tulum - the OP mentioned Tulum. The archeological site and town of Tulum itself is very approachable on your own if you choose. Very easy to get to and well developed. Tour vs individual - really your preference for flexibility of having your own transportation and being able to explore as you wish or having everything organized for you but sacrificing some flexibility.
Riviera maya is a fantastic destination full of wonderful places to explore with lots of options about how to do so. Pick the choice that is best for you and your party.