I would not recommend Xulum. It's all show IMO. In addition, it is a bit outside of the 5th ave pedestrian area which could dampen a festive mood.
Como Como was excellent but note they may be very challenged to host a party of 10 since the restaurant only seats about 30 total.
Luna maya sounds ideal for you. There are plenty of places on and around 5th with mediocre food and music for your large party.
Thanks to all. Here is a trip report as promised.
100% Natural (near 5th and 10th) - Lunch - fresh fruit and smoothies in a beautiful oasis off of the bustle of 5th Ave shopping. Vegetarian pita with hummus and a thick sour yogurt was a standout. Service was very slow but we were in no hurry on a hot day.
Alux (dinner) - As advocated by Veggo, it is not often one gets to eat in a cave which certainly made it memorable and worth the experience. The cave is amazing. The meal itself was forgettable and pricey for the quality. All dishes we had (4 total) were blah to just OK. Drinks were weak and uninspired.
El Tabano - Lunch in Tulum - Exceptional!! What a treasure. We went out of our way to eat here twice. An elderly women from Veracruz runs the simple kitchen in a tropical garden setting. Herbs are drying in the sun and complex aromas fill the air. The menu is written on a chalkboard by a very laid back staff. The fact that my 5 year old asked the waiter if he was Jesus and my wife's first impression was that they didn't seem to know if they were open pretty much sums it up. But on to the food... Standout dishes included a cold cucumber, avocado & mint soup, meatballs in a spicy cinnamon bean sauce and a beautiful chile stuffed with mildly spicy mixed vegetables and cashews for texture. All washed down with the best margaritas I have had (a spicy cucumber).
Luna Maya - Dinner (5th and 38th) - This restaurant has a beautiful setting overlooking 5th Ave. The outdoor seating is slightly better suited for lounging than eating. The wait staff was very knowledgable and attentive. We had tinga chicken tostadas as an appetizer and it was absolutely fantastic (A+). 2nd course was a hibiscus flower & goat cheese salad (B+) and an ultra smooth black bean soup with goat cheese and tortillas (A). 3rd course was a red snapper with midnight black rice and bananas (A). Dessert was a moist corn flour cake (B). All dishes had beautiful presentation.
La Cuerva del Chango - late B-fast (near 5th and 38th) - Frustratingly incompetent staff and the fact that they only take cash dampened what would have otherwise been a wonderful B-fast. We had fresh squeezed juices, scrambled eggs with an excellent mole sauce, fruit crepes with yogurt and honey, fresh toast with cinnamon and plum compote, wonderful homemade naturally flavored devils food cakes.
La Tarraya - dinner (5th and 2nd) - This restaurant was an easy walk via the beach from Playacar. It was a very good meal for a great value (900 pesos for 3 adults and 3 kids with drinks). Our feet were in the sand, the beers were cold and a guitar player had my kids happily dancing under a palm tree. Not much choice here but the fresh fish of the day (grouper for us) can be prepared 4 or 5 different ways. I enjoyed the Tarraya sauce. Calamari was fresh and tasty though a bit soggy. The chips and salsa were addictive.
Xulum - near the entrance to Playacar Phase I - Just got takeout from here because it was easy. This establishment is obviously a bit of gimmick and 2 out of 3 of the dishes were lame. I have to admit that the BBQ beef tacos were quite good (A). Alas, I chose not to try the grasshopper or other insect tacos. My kids liked conversing with the parrot.
Como Como - dinner (5th and 39th) - Quant place (maybe 30 seats). Very nice wine list. Flavorful bruschetta, lentil and quinoa salad (A+), smoked salmon in filo (B), spinach raviolis in a truffle oil (unfortunately B+ for being a bit overcooked) and a homemade spaghetti with lobster (A) only because the portion was a bit small.
So, what do I think I will remember in 10 years? I would say, in order, El Tabano for the beautiful food and setting, Alux for the setting and Luna Maya for a very nice and romantic dining experience.
Thanks again to all for the recommendations!