I totally agree with Robo and Kim. Take the ship's tour to Tulum. They will only let you off in or for tendering to Tulum if you have bought the tour. Save your souvenir shopping for the craft booths at the historic site. You can barter with the vendors. Once you tender back to San Miguel, you can shop, go to the beach; Playa del Sol is a $12 taxi ride each way or Chankaanab Park as mentioned. The ruins at San Miguel are not as vast and exceptional compared to Tulum and Chitzen Itcha is much too far of a trip unless you fly $$$.