I'd wear something comfortable! After the ferry ride to the mainland, you have a brisk walk through town to the bus staging area. Then you board the buses for about an 90-120 minute ride with a stop at a jewelry mfg (caveat emptor). When you arrive at Tulum you can opt for the tram back to the ruins or walk. Like a previous poster mentioned, there is some climbing around so you might not want sandals w/o a rear strap. Don't miss the beach beneath the cliffs of the ruins. It'll be HOT!!! And like I said, there'll be PLENTY of IGUANAS!
PS - Take plenty of SMALL BILLS for shopping in the souvenier area. Several passengers on our bus bought postcards with $20s and never saw the change.