Thanks to all (and Veggo especially!) for the recommendations for our trip -- we ate very, very well. Here's the lowdown:
We stayed at the Mandarin Oriental, which was perfect pampering but expensive food. Had tiraditos and ceviche for lunch at the hotel's beach restaurant (called Ceviche, so there's a clue about what to order). Snapper tiraditos were excellent; ceviche was overly lime-y (as in drowning), guacamole was truly wonderful. Expensive, compared to what we discovered elsewhere. Dinner the first night at Ambar, which everyone seemed to love except me. I must have ordered badly -- can't remember the first course, which is always a bad sign, and second course of porcini-crusted tuna, which was OK. If you're staying at the MO, I'd recommend eating there on the first night so that you don't realize how comparatively expensive the food is.
Day two: more tiraditos on the beach for lunch. For dinner, we went with the concierge's insistent recommendation of Yaxche. The menu is enormous -- so long, in fact, that it's hard to make sense of it. Excellent shrimp ceviche (but not nearly as good as the ceviche at Cactus on our last night), good-but-not-great food otherwise. I couldn't make sense of the menu so I ordered the Maya platter, which had some chicken, some beef, and some pork. Chicken was good; rest was OK. Pleasant garden, nice server.
Day three: La Floresta for lunch. Wonderful, wonderful shrimp tacos and dang good ceviche mixto. Even had small oysters in it! Dinner at Negrosal, another concierge-driven choice but a much better one. Good food, good (but somewhat pricey) wine list, very nice people. Even made it to dessert for the gooey chocolate souffles. My son tried the "strawberry deer," which turned out to be venison in strawberry sauce -- tasted a little like meat with a big strawberry daquiri poured over it. Not our fave, but the rest of the meal was excellent -- grilled lobster, grilled fish.
Day four: Not much to report -- more guacamole on the beach and dinner at the MO's other restaurant, Aguarmarina, where the fish tacos were delicious (pricey, though!).
Day five: down to Tulum, where we all fell in love with El Tabano. I would eat there every day if I could. For those not used to Tulum (like us), it's not in an obvious location -- go toward the hotel zone and keep going. It's basically a big open-air hut, with interesting books, kind and solicitious people, a Rube Goldberg-like sculpture at the water faucet outside the bathroom, and my personal favorite food of the trip. And amazing value.
Day six: lunch at El Fogon. Another amazing value, and great food. Guacamole, beans, arranchera, and carnitas. We ate like pigs and rolled out very happy.
Final meal: We wanted to go to La Terraya on the beach but it literally shuts down at 9 pm. So we tried Cactus instead. Had the best shrimp ceviche I'd ever had, in a remarkably generous portion, too. Others had great chicken soup w/onions, avocado, and lime, and excellent nachos. The baby squid with potatoes (very Basque-like) was excellent. The pork in mustard sauce less so. For dessert, the flambeed bananas were awesome, the crepes OK, and the brownie not really worth it. One other quibble -- it was surprising that a restaurant flying a French flag didn't serve wine!
We can't wait to return and try the other recommended places, once we regain the courage to put on a bathing suit after all we ate! Many thanks!!!!