Collectivos are very affordable methods of transportation. However, they are not usually the most convenient. Often times, riders may be left with LONG walks from the highway into their destination. Plus, they stop often - whenever someone flags them down. If you've got the time and don't mind a lot of walking, give them a try.

Of the three destinations you mention, Playa del Carmen would be the easiest to reach via Collectivo. But, it's probably not all that expensive via Taxi.

Coba might be tricky, as you've got a couple of highways involved. Might have to go into Pueblo Tulum and buy a ticket on one of the bus lines operating from the terminal south of town (or go into Playa and do the same).

One more suggestion would be to avoid the Collectivos on Saturdays. That's when many of the workers in the area try to get home for their day off and those vans are packed VERY full.

If you're going to do a lot of traveling out and about, perhaps a rental car would be a better alternative?

Dave