We are going to be at the Royal Playa del Carmen from 3/8 to 3/14. We have rented a car,and we plan to take several excursions (Xcaret, Xel-Ha, Chichen Itza, etc). My question: is there an advantage to paying for a "tour" of any of these places from one of the many tour companies in the area, or can we just drive there ourselves and still get the same experience? Many thanks!

You can hire a guide at Chichen Itza for a tour of that site. I don't particularly like the large bus tours because they all arrive at once and you have to stay with the group wherever they go. Drive yourself and hire someone when you get there.

I wouldn't hire a guide for ruins; just read a guidebook or the brochure you get at the gate. You certainly don't need a guide for anyplace like the "eco"parks. By the way, you might not want to do both of them; one is a lot more touristy (Xcaret) than the other (Xel Ha), and they are expensive. But a guide for those would be like paying for a guide at 6 flags.
Drive yourself (I'd do the toll road at least one way to Chichen [$20 US]; leave early and get anywhere early to avoid crowds. Don't forget to bring water with you (save a LOT of $$).

If you are in Tulum it is a long drive out of your way to the toll road up by Cancun. Take the Coba road west from Tulum to Chichen Itza. It's free and is an easier way to get there. You'll also get to go through Valladolid and go past the prison if you want a hammock. The only reason I suggested the guide at Chichen Itza is they are so informative and make the tour a little more than just reading it out of the book.

You can hire local guides for a low price when you get to the ruins. I know I enjoy hearing about all the local history.

I am going to the Yucatan for a week at the end of February, using Playa del Carmen (Hotel Shangri La Caribe) as a base. We have rented a car, and I have a couple of questions about driving and tourist attractions...

First, how long does it take to drive to Chichen Itza from PDC, are the roads OK, and how early should one arrive at Chichen Itza in order to avoid the worst of the crowds? (thanks for the advice on hiring a local guide, which I'll do!)

Second, I want to visit an eco-park. Am I right that Xel Ha is a bit less crowded/touristy than Xcaret? Does anyone know what a simple entrance ticket to Xel Ha costs? The website only offers an expensive "all-inclusive" ticket that includes meals and snorkeling equipment rental.

Greatly appreciate any help on this, as well as any other suggestions for day trips from PDC!

IMO: The only thing "eco" about the two X- parks is not using sunscreen (except the paba-free that they sell to you; remember to bring your own reef-freindly screen, or prepare to get soaked, unless you just keep your teeshirt on to avoid the sun).
I prefer Xel-ha, a little less touristy and a little cheaper, but I don't know the current rates.
I suggested the long highways and toll road instead of the Coba-Valladolid route because of the much slower rate of speed and the topes. It is certainly a shorter distance to drive the diagonal, and if you wanted a long day, you could do Coba on the way back.
I think guides to these places are a waste of money, and I would rather follow my own agenda. But I read about places before I visit. (A tour guide for my first trip to CI was totally focused on blood sacrifices and ways to put needles through mens' "nether" parts, and didn't want to walk anywhere like the cenote or inside the pyramid (too hot); and the guide I once payed for at Coba dodged the "officials" and spoke neither English nor Spanish; but it was fun hearing all the Mayan words).

Tour guides are overrated, I was in the PDC, Tulum, Cancun area and we did our own thing, paid less, wandered around on our own and enjoyed ourself more than we would of, if we had been herded around like cattle. I recommend Tulum as well, it is close to PDC and is beautiful, the beaches are great too.