Hola! A great option for an inexpensive excursion/experience for your guests is to take them to Akumal for a day. You can hang out at Half Moon Bay and snorkel with beautiful sea turtles. It's just 20 minutes from downtown Playa. Depending on the number of people, you can rent a van or bus or take taxis. Once there, there is a convenience store for drinks and snacks and a nice open air restaurant. Renting snorkel equipment for the day is about $15 for mask, snorkel and flippers and your guests can take turns going in so they can share.
The turtles aren't far out or deep, it's an amazing experience! ~ weddingsinplaya