I have been to Playa Del Carmen, Cancun, Cozumel, and Puerta Vallarta. They last time we went was in October 06', we stayed at my favorite place of them all, was the Seaside Suites (
http://www.karismahotels.com/seaside/), they are south of Playa Del Carmen by about 20 mins, my wife and I are both divers, but there is plenty to do there if you arent a diver. They have hand painting crafts everyday, lot of amenities, and of course the beach with, pretty good snorkling. Your are also real close to the Tulum Mayan Ruins (hint: dont book a day trip there, take a taxi and pay the $3 entrance fee), they will try to sell you trips everywhere for it at $80 per person. There is also Xel Ha, its a natural water park, we did Xel Ha and Tulum ruins in the same day. Very fun.
My wife is still fairly new at diving is gets nervous each time. I used Dive Mike (
http://www.divemike.com/), awesome prices, awesome service. Juan was our dive master and he was great with helping my wife get calm. Earlier this year she had a terrible expeirence in Puerto Vallarta and almost gave up diving. If I am ever in the area again, there is no one else I would consider. Its located in Playa Del Carman.
Note: I really like to stay out of the tourist areas, so Cozumel wasn't my cup of tea. Its good to go for a day, shop, and get out. I like to go and relax, and dive.
Another thing that might help, Deep Sea Dectives are doing a show this week about the Cenotes in Mexico, and talk a lot about the Mayan history, very interesting. Try to catch a repeat of that episode.
I have a wealth of Mexico knowledge, so if you have more questions feel free to ask.
Edit: I forgot to tell you about the best part of the resort, all inclusive, top shelf liqours, and spa tubs in each room. 24 hour room service included, also if you want, tell them what time you are going to back to you room, and they will have the tub ready for you. Including bubbles and rose pedals. Your wife will love it.
Make sure you book through one of the online services, they are way cheaper then going directly through them, I also found a link that took 10% off the rooms, not sure if it still on that site, but let me know and I can track it down for you.
Another quick note: We spent just over $2000 for 7 nights for flight/hotel, and that was for Ocean Front, if I would do it again I would do Garden view, you don't really get that much more for ocean front.
Joey