I just got home from a week in Playa del Carmen, and would certainly echo the recommendation to head there. 5th Avenue there at night time is lots of fun...lots of great food and music. From there, you can take a quick ferry ride of about 40 minutes to Cozumel, where there's some great snorkeling. I also spent a day at Xel ha and Tulum and would definitely recommend both. Tulum isn't quite as spectacular as Chichen Itza, but it's easy to get to from Playa del Carmen and only 15 minutes from Xel ha. I thought it was cool to see them still excavating the ruins there and the spot by the ocean is gorgeous.