I believe that one of the biggest draws of Cozumel is its beautiful reef, so if your friends don’t dive, they ought to at the very least take a snorkel tour. There is some great snorkeling in the shallows off of Punta Sur Park and at Columbia Shallows. But there are a ton of other things to do on Cozumel. I heartily second the notion of an East side tour - Rasta’s Bar, Coconuts, and Mezcalito’s at the turn off to the trans island road for some great grilled shrimp. Then there is the tour of the Kaokao Chocolate Factory, haggling with the locals over a hand woven hammock or a molcajeta at the Municipal Mercado, Breakfast at the Museo restaurant and a tour of the Museum, a day at Chankanaab Park. Lots to do.