Hey Pia, glad to hear your having a ball, if I may make a couple of suggestions. Merida is a beautiful and surprising city that I'm sure you will enjoy. The old Cathedral next to the central plaza is very old and very interesting, don't miss it.
On the road from Merida to the port of Progresso (35 kilometers) there is a fantasic area of Mayan ruins (can't remember the name but is very popular) that you should take the time to see. Don't get hustled by the "guides" at the entry, just get the brochures and do a self guided tour no matter how these guides try to insult you!
If it was me I would skip "Isla Mujeres" and opt for the island of "Cozumel". When you get there rent a car or open air "thing" and get the hell out of the main city as it's just a repeat of every tourist area in Mexico with the tired old "Carlos O'briens," Senor Frogs", drunk and obnoxious loud Americans etc;, etc;, etc;.

Drive away from town and go to the Palancar Reef Gov't operated beach (I forget the name but you can't miss it (something like "Chankanar"). It has incredible "snorkeling" and you won't want to get out of the water, I promise! You can rent all the scuba or snorkeling equipt there and they also sell beer,food, and have chairs under the palms in the shade, beautiful!
Then drive about 20-25 kilometers further on until you come to the point where there is a place that's called "Bob Marleys".(open air, grass roof) It's almost deserted here and they sell beer and great food and the ocean is incredible here (waves and great beachcombing) I have known the owner for many years since he was a child and everyone is friendly and fun. Kick it here for a few hours listening to Regaae music, drinking a few beers and sleeping in one of the many "hammocks" on the beach.
Travel on down this windward side of the island and there are a number of small places to eat or have a beer (very funky!) Many lonely deserted beautiful beaches here. Veeery Romantic! de tu gusto!
Have a ball!