I second Tulum, but as well have a little bag with few things and perhaps stay the night if you long to. Take a bus to Tulum Pueblo and then grab a cab to the beach and visit the many cabana hotels. We always stay at Zazil Kin (best beach area that I know of with the coolest bar) for a few days. Cabanas are on avarage $30 a night and even cheaper. There are nicer and more rustic places to stay as well along the beach. It's a cheap little side trip even if you're still paying for your hotel back in Playa where the better half of your things are. As well Zazil Kin and several other places are just a short walk down the road from the ruins. I gotta say it's a lovely sight to be swimming in the ocean there and look down the beach and see the pyramid.
Another favorite thing that we do is a quick trip to Cozumel and we rent a scooter and cruise around the island and find ourselves a secluded spot on the beach. Car rentals are cheap too. We stay the night, but one doesn't need to for the ferries run late enough.