I agree with GD... It's hard to beat whole grilled Red Snapper (called "Chillo" here). You can purchase whole or filleted Chillo, Mero/Basa/Sea Bass , and other firm-fleshed fish at most supermarkets. I just grilled a fillet of Basa I purchased at La Sirena (RD$ 59 pesos a pound). Simple olive oil, spice rub, a little limon and direct on the grill...no foil wrap (although that's a great method too, especially if you have fresh herbs). It was quick, inexpensive, tasted incredible, and muy saludable!!
Another great option is grilled shrimp...buy the larger (16-20 or 21-25) raw shrimp at the supermarket (usually around 200-350 pesos/lb). Marinate them for 20 minutes in just about any marinade, salad dressing, fruit juice, liquor, etc. (not too long or they will "cook" in the marinade, like ceviche) and put them on skewers...add pineapple, onions, and peppers if you want, grill for about 2-3 minutes...super quick, healthy and delicious...