My parents (late 60s) and I just returned from six days in Cancun/PDC -- it was the first visit for us! Thanks to everyone for posting their suggestions on this board -- great info!
Me: Love street food and unique/different flavors. Somewhat adventurous (wanted to try chapulines but my parents nixed the idea). Pet peeve is overpaying for what you get -- I'm always looking for a good price/quality ratio for meals. Relatively familiar with Mexican food as I often eat at hole-in-the-wall authentic (few gringos) places in the Latino neighborhoods of the San Francisco area.
Parents: Middle-of-the-road diners but open minded for trying new things if it's not too weird. Very little exposure to Mexican cuisine -- Chevy's/El Torito is about the extent of it.
PLAYA DEL CARMEN
LAS BRISAS: We a decent inexpensive seafood dinner here -- the seafood combos for two are a great deal, I think we got a big platter of fish, lobster, squid and shrimp for less than 400 pesos. However the restaurant is a bit dingy (our tablecloth looked like it hadn't been cleaned for a couple days), and while the freshness of the food was okay, it wasn't remarkable. However a good choice for a inexpensive seafood meal just a block off Fifth Avenue. Mixed seafood platter for two, two beers, agua fresca, and extra garlic sauce: 525 pesos including tip.
CARBONCITO: This was the most "touristy" place we went in PDC; we ate here for brunch. We thought it was okay -- a solid safe choice but nothing special. It bugged me that the agua fresca in Spanish was 5 pesos cheaper than if you ordered it in English. Two breakfasts, one aqua fresca: 290 pesos including tip
LA PESCA: We had dinner here the night after Las Brisas. Definitely more expensive (by about 50%) than Las Brisas, seafood was slightly fresher and tastier but not by much. We had the mixed ceviche (which we didn't like), lobster with shrimp combo, and fish fillet Veracruz. Overall it was good but not great; perhaps we should have gotten a whole fresh fish instead. Dinner including one beer, two limeades and automatically added tip: 750 pesos
TACO/TORTA STANDS in the plaza at Juarez & 25th: Good cheap food -- I think a torta was only 20 pesos! We had a really tasty lechon torta, and a decent cochinita pibil torta -- make sure to add lots of the pickled red onions as it makes a big difference! I thought it was interesting that the torta bread we got there was different than the torta bread used here in the San Francisco area -- in Playa it's somewhat long and thin, almost torpedo shaped, whereas in SF it's always round kinda like a big burger bun.
PALETA (Mexican popsicle) store on Juarez between 10th & 5th Ave: So many interesting flavors (better know some Spanish, though)! I really loved my pineapple/chile flavored one. Avoid the ones that just taste like cream unless you like really subtle flavors. I think getting paletas is a MUST DO in Mexico -- even here in SF area where there's a huge Hispanic population, I don't see many standalone stores (lots of small carts trolling the neighborhoods, though). There are a bunch of other paleta shops around, I'm not sure if there's one that's particularly better than the others.
SEAFOOD RESTAURANT AT XCARET: We got a package with an included lunch buffet (good thing to do as the lunches a la carte are 370 pesos/person, but you can eat lunch up til 5pm). My parents voted for the seafood buffet -- unfortunately (but maybe not surprisingly) most of the seafood there didn't taste fresh. Avoid the whole fried fish and the crab soup! However the seafood ceviche was decent. If I went to XCaret again, I'd pick a different buffet.
EL FISH FRITTANGA: All the locals (guys at the hotel, taxi driver) recommended this to us so we had high hopes. We had 6 pescadillas (thought the ones at XCaret were actually better), 3 octopus tacos (they looked so good being cooked at the takeout stand up top but were nothing special when we ate them in the restaurant), the "shrimp in orange sauce" which our waiter said was a favorite of the locals, and 2 limeades. While we wouldn't order the orange shrimp again, I was happy we tried it as it was very different from anything I've had before -- it was almost Asian in flavor, with a LOT of mayo (maybe that's a local preference?). Lunch for 3 was $35 including tip.
LABNA: My favorite restaurant of the whole trip. Going to downtown Cancun was a bit of a hassle, though -- it was almost an hour each way on the bus from the Westin Regina! We also were steered the wrong way by a policeman and some others and got pretty lost looking for it, with my parents freaking out about our safety the whole time. Fortunately we arrived with no incidents to a restaurant which was strangely almost empty our entire dinner. I loved the pumpkin salsa served with the chips. We ordered two lime soups (you MUST put the extra lime skin into the soup else it tastes pretty much like limey chicken soup), the "Yucatan Tour" sampler, the cochinita pibil, a beer and a pitcher of limeade for 534 pesos not including tip. The poc chuc in the Yucatan Tour was stellar, and I liked some of the other items in the sampler (though I wish the server explained to us what they were). The cochinita pibil was nothing special, but I'd visit this restaurant again to try other offerings. There were also lots of tasty-looking food in the park stalls and streets, but I didn't want to freak out my parents more and so we didn't try any.
MEXTREME: I wanted to have lunch in Punta Cancun so that we can look at the touristy places there, and this was recommended by both the MapChick maps and TripAdvisor (the latter saying it was a good and inexpensive choice). Our lunch here was the priciest (slightly) of our entire trip at 760 pesos (including tip) for a mixed grill (serves two), a large beer, and a large cocktail. My parents liked the mixed grill (steak, chicken, shrimp, veggies) but I thought the steak/chicken fajitas at Chevy's are much better than the meat here -- the chicken especially didn't taste good. I appreciated that they had cactus on the mixed grill, but otherwise I'd say this is a place for the non-Chowhound crowd.