I to went to Mr. Sanchos beach. It was so fun. They have a little flea market, a bar, a small freshwater pool, jet ski rentals, and the PRETTIEST water.
As soon as you get off the boat, follow the signs through the pedestrian mall to the cabs and there will be a ton of people there waiting to take you wherever. Our ride to Mr. Sanchos was right at $10 and there at taxi's at Mr. Sanchos readily available to take you to whereever.
Once you leave there, take a cab back to San Miguel and go to Senor Frogs and Carlos and Charlies. It's right by the pier so you can drink and walk back to the boat. Senor Frogs has a tequila conga line and they give you free shots. It was a lot of fun.
If you're not into the whole drinking scene then the pedestrian mall (not the 2 story one that's right when you get off the boat) has a lot of JUNK (and I literally mean junk) for sale. There's tequila STORE there and LOTS OF JEWELRY STORES. Lots of silver.
Also the worlds SMALLEST Hard Rock Cafe is in Cozumel right down from the pier. I live near Orlando where the worlds LARGEST Hard Rock is, so going to the smallest was kind of fun.