Haven't done Dunn's River Falls, but would like to since it's my last name. We have done the Ruins at Tulum. It is a fairly long bus ride and they stop at a little roadside tourist trap to try and sell you their goods. We made a pretty good deal on a hand made tablecloth. The bathrooms, which you need by the time they make the stop are deplorable, you would be better off finding a tree.
The Ruins are pretty fascinating, the day we were there it sprinkled pretty much the whole time we were there but it was still a part of history you won't experience in too many places. The view from the highest point overlooking the ocean is the most beautiful water I've experienced.
Before the ride back we had lunch at a little mall area where the buses unload and is obviously authentic Mexican, which is great if you like that kind of food, of which I do.
My take, I'm glad we did it once but I don't think I would revisit it. Do it once, you won't regret it!!