Hi Veronique,
Isla Mujeres itself is very quiet and relaxing. The beaches are beautiful and have plenty of barmen attending the deck chairs etc. Even a man selling coconuts! There are plenty of bars, shops and restaurants. I don't remember names but if you meander around you will find one you like. I tended to stay in the hostel as its very lively and has its own bar/restaurant. As for activities...The hostel is fantastic and organises trips daily. I know some people did a diving course on the island also and there are dive shops where you can organise snorkelling/diving. There is also a turtle farm at the south of the island which explains conservation efforts in the area though I think I may have visited at the wrong time of year as there were only a few tanks occupied and no nesting. Also I would advise you to bring planty of bug spray as I got bitten quite a bit in Isla.
Pocna is one of the beat hostels I've ever stayed in. You'll have a fantastic time!
Take care,
Yvonne.
PS Have also been to Playa, Merida, Tulum, Cozumel and passed through Cancun.