My husband and I will be staying at an all inclusive in Playa del Carmen in December. We would like to take an excursion one day. What would you recommend and is there a specific tour guide who is excellent?

What kind of trip did you have in mind? Visiting Mayan ruins, snorkeling, shopping, nightclubs like Coco Bongo, fishing, etc.
Lots of stuff you can see just by taking a taxi or colectivo, like Tulum ruins or snorkeling in Akumal. Other stuff that is further afield you can see with a tour like Chichen Itza or maybe Coba ruins (my favorite, along with Ek Balam).
We always had a rental car but there is a company called Yucatreks based in Playa del Carmen that gets good reviews for both small group (12-16 people) and private tours. Another one is Edventures, I've met Eddie back when he was just getting started and he also gets good reviews.

We have decided on a private Tulum and cenote tour. Can you recommend a specific tour guide?

Look into Monica Rendon at Soy La Pesera tour agency. She is great.
https://soylapesera.com/?fbclid=IwAR...OWnWsn88DQtaMo

I've done that several times from Playa del Carmen, but never private if you mean you are the only one with a tour guide. I actually like being in a small group and meeting other people, I've met some very interesting people that way, actually, as that's a pretty wellknown resort area and people come there from lots of other countries.

I've been to all these places, Tulum, Ek Balam, Chichen Itza, Rio Lagartos, etc, using tour groups.

FWIW, this is the main company I use, they are excellent. It says Cancun but they have tours from Playa also. All the tour guides have been excellent on the ones I took.
FWIW this is it
https://www.cancun-discounts.com/

They will arrange private tours also, although they seem to be packaged itineraries. Still, their prices are very reasonable for that. Looks lie it is mainly the transportation they are arranging for you, maybe they talk en route, the guides on the ones I took did, but then there was a different guide at the sites, a local. In fact, it could be there are even regulations about that, wouldn't surprise me, but I don't know. Both the El Balam and Chichen Itza mention you having to pay for a private guide at the site, if you want one.
https://www.cancun-discounts.com/pri...ncun-tours.htm