The beach restaurants we really liked were:
Casa Banana (near km 7.8) – pretty much everything we had here was awesome and we had dinner here twice. They use a wood fired grill to cook their dishes and also make house made bread. Their cocktails are both strong and tasty; the caipirinha and mojito I had here were made from very fresh ingredients. My favorite dish was the wood grilled fish, which came with roasted sweet potato and roasted cauliflower. It is big enough for 2 so if you are not super hungry; I would suggest sharing it.
Simple (near km 8) – they have fresh fish every day and we had lunch as well as dinner here. For lunch, we had a fish ceviche which came with like a pound of fresh fish marinated in a lime based sauce. For dinner, we ate a freshly caught grouper which they will happily split into two different preparations (spicy for my boyfriend, non-spicy for me). The owner is also super friendly and will offer you some mezcal that is pretty much the definition of firewater.
El Tabano (near km 7) – their cerdo pibil is very rich and the pork leg is like butter. They also have homemade bread, which is served with a savory sauce that kind of tastes like spicy hummus.
There are quite a number of really good restaurants in town (all are on Avenida Tulum) and they are very cheap:
Pollo Bronco – I love this place so much. The staff here are really friendly and the chicken they serve is moist and flavorful. You can get a half chicken for 80 pesos, which comes with salsa, rice, and tortillas.
Flor de Michoacan – We came here literally every day to get popsicles (15 pesos). They are made from chunks of fresh fruits like watermelon, coconut, pineapple, guava, etc. We really should have something like this in America. It’s nice to get a popsicle and sit either in the shaded area in the back or in front and people watch.
Antojitos La Chiapaneca – The tacos here are so cheap (7 pesos without cheese and 10 pesos with) and so good. Their specialty is al pastor and they cut the pork from a rotating spit. Beware that they are only open at night.
El Camello Jr – They serve shrimp tacos that are really fresh and come with a slice of avocado and lettuce. They also have nine different kinds of hot sauce on each table so you can pick your spiciness accordingly.
Alex – this fruit store is very close to El Camello Jr. I bought the sweetest pineapple of my life here. We also got very sweet bananas, green oranges, and peaches here.
At Akumal, we ate at Cueva del Pescador, which is along the strangely winding public beach access path from the parking lot to the beach. The langoustines here are extremely flavorful and the limonade is perfect after swimming with the sea turtles.
Lastly, it is really worth it to walk around Tulum a bit. There are a lot of interesting stores to pop into and graffiti on the walls of buildings.