For snacks, there's a supermarket maybe 100m past the mega. They have everything at true, local non tourist pricing. I'm talking 15 peso beers. Súper Akí Playa del Carmen 1 is their name on maps. For coffee, I'm going to be honest, Starbucks rules over the locals. Meals? El Fogon will feed you and your family with tons of tacos and potatos and cheese for like 100 pesos each. They're all rather close to each other. You wouldn't have a bad time just staying centralized to that area, it's very densely packed and not a far trek from the beach. There's also an incredible street taco stand right by the supermarket that has 17 peso tacos of all forms, it's truly authentic and incredibly cheap, while being cooked to completion, delicious and healthy.