If you are looking for 'not fancy' and 'lots of do-nothing', I think Isla Mujeres will suit you fine.

We took a day trip there last year while vacationing in Cancun and we really enjoyed it. It isn't much, really.....only a few hotels and restaurants, not too many easily-accessible beaches....but it was a fun place to spend a nice, sunny afternoon.

We rented a golf cart to tour the island on, and had fun navigating our way around. Beautiful views all along the islands perimeter.

We were disappointed in the turtle farm, however. If it had 20 turtles, I'd be surprised. Nothing at all like the turtle farm on Grand Cayman that we'd visited the year before.

I hope this helps. We only did a day trip so I don't know how much activity you'd find if you stayed for, say, a week.