I go to Coz in the summer to cool off. It just doesn't get all that hot on the ocean. Inland ruins on the peninsula yeah, they are roasting, but not on the water.
Since I am cheap, lazy, and don't want to have to work at making dinner plans with others, I like the All Inclusive approach. Like at Hotel Cozumel, it's very nice to have a good breakfast at the hotel before diving and a good lunch there on return. They were all at the dockside cafe last summer. Then for supper, go into town if I feel like it and skip the free dinner at the hotel, or eat there - choose day to day. I do hate paying a taxi fee alone, adding $10 or more to my dinner costs, but then I also hate feeling like I must get myself invited to tag along as the third wheel with others at the hotel going into town. Nice to keep the options open.
3 weeks
anywhere would be too long for me. I know that others feel differently, but if you're going to be there that long - mainland cenotes & ruins are good ideas, true, and I'd want to stay on that side of the channel for part of the stay. Multiple ferry rides can get tiresome. Playa del Carmen is not as safe as Coz I don't think, but it should be easy to stick to the safer areas - and then there's Tulum and other places to relocate over the adventure. I really enjoyed staying at the delightful hotel at Chichen Itza's back gate, then a nice breakfast and walking in right at opening time - hours before the buses arrive. If you like ruins, Ek Balam is wonderful and nearby too; if you don't, ok stick to the coast.
Whale shark snorkeling is wonderful and I'd love to do it again. Some will do a day trip from Coz but gawd what a long day, and then only one day of snorkeling with the Dominoes. Holbox is a delightful island to visit and go from there, or many snorkel boats leave from the Cancun area to swim on the other side of the herd.
You might plan on flying into Coz, then out of Cancun, with only one ferry ride - maybe?
Now, staying on Coz for an entire visit is really simple, but exploring the mainland can get much more creative. If you're thinking of the latter, we might suggest a number of ideas. For now, quickly order several of the Can-Do maps/guides:
cancunmap.com-Maps and travel guides.Your information source to Cancun,Cozumel,Isla Mujeres & Riviera Maya