Doing this is really easy. Getting to Playa del Carmen shouldn't be a problem. There are minbusses that go all the time pretty cheaply. Though if you're travelling with luggage it would probably be MUCH more comfortable to arrange a taxi or take a coach bus down. Once in Playa del Carmen you can fly over pretty cheaply, or take a short ferry over. Cozumel is OK, but IMHO there's nothing that great about it, San Miguel is touristy and small, and they don't have much in order of beaches (unlike the Cancun/Riviera Maya side of things). Great! diving out there though.

Although touristy, I would recommed spending at least 1/2 a day in Playa del Carmen. Its absolutely overloaded with tourists every day of the week, but once you get past that, its actually got a lot of charm and a lot of really cool boutique hotels, bars, and restaurants. A personal favourite of mine is the Frida Bar (which has incredible decor based off of the Mexican Artist), its on Avenida Quinta (5th Avenue). Also lots of opportunity for gift shopping, especially as you head south of the ferry dock (there's a conglomeration of shops there where you can bargain hard, and get things pretty cheaply).

Best

W