I’d like that. A lot. 
I never tire of ruins, pictures of ruins, or visiting ruins …
I’d also second the suggestion of driving around - if we can do it anyone can! The caveat is that it isn’t cheap to rent a car, and you have to get used to the oddly random nature of Mexican speedbumps.
The Mexicans we interacted with in Yucatan seemed very friendly to us, we never had any problems.
Xel Ha does serve alcohol…we had a beer with lunch. Try Modelo Speciale in the stubby bottle (NOT the dark one). The water was cold when we went but, like you, my tolerance is very good.
Uxmal was beautiful. They get very little traffic and it 's a beatiful site. I can’t remember if it would be wheelchair accessible.
Cancun is about an hour away from the Barcelo in the Riviera. There is a place called Casa Cenote which is beautiful for lunch. Last I heard, used to be owned by a Texan and the place hops on a Saturday night but nice and quiet for sunday lunch. They have a website. Definitely worth the trip for the view and the swimming in the cenote - it’s very large; more like a river, really.
In Merida, I believe the festival is Saturday night. It’s very crowded there.
Cabs are easy to hail BUT MAKE SURE YOU AGREE TO THE FARE BEFORE YOU GET IN! The local hotels will post a suggested fare to different destinations. Use it.
Check the website I gave earlier unofficial-barcelo-maya.com - it promotes the hotel however gives lots of information about the area and things to see.
I didn’t do the swim with the dolphins thing - it’s actually very sad to see them penned.
Car rentals - make sure you check EVERYTHING on the car over before you pick it up including the tires, windshield wipers, everything. And make sure you go 'round the car with the fellow when you bring the car back. Please read the rules of the road before venturing out, especially the left turns. They are quite different than here, especially on the highway (1, I think?). Speed limits don’t matter. Neither do stop signs in town. Beeping your horn is a national pastime. Pay attention to the way the bus drives on the way to your hotel or if you take a tour, along the highway. It will be important when you are driving. A Chevy Pop is the smallest car. Quite interesting, that. Stray dogs are many. Be wary of them, they are not pets. Get used to seeing them dead on the road.
Should anything happen, wait for the policia in the, I believe, green outfits. They will speak and understand English better than the locals and are special for the tourists.
Make an effort to speak Spanish, even if it’s not perfect. Treat everyone with a smile and you will have a wonderful time.
Oh, for bartering, start at 50% of the asking price and go up. They are used to it.
And watch what you’re buying. In the Riviera, hammocks; painted wooden masks and jaguars are local and beautiful. Also cotton clothing. Vanilla and tequila as well. Do not buy mesquel unless you like rough liquor. Gold tequila and up is quite good for sipping with sangrita.
Caribbean lobster is not as good as Maine or our east coast lobsters - a little tougher.
Drink Micheladas or chiladas (without the spices). It’s very good. The fresh limes are wonderful - put the juice on everything.
Take American money and pesos. Everything is sold in -equivalent to American dollars- anyway.
Wow thanks you guys for so much more information. I can’t wait to try Negra Modelo in Mexico (I like the Especial too, Canadiangirl, but I lurves me my dark beer!).
James is really comfortable in the water and is really excited about Xel-Ha (he rides everything at Disneyland except for Indiana Jones and Splash Mountain, only because they jostle him too much - for physical things, jostling around and walking/climbing long distances are the only real concerns for him).
I think we may take a day or two to drive around and just take things in at a relaxed pace.