Summer squash varieties work really well in this dish, and are very typical vegetables in Mexican cooking. Choose the freshest or your favorite variety.
I grill my corn first, but this is not a required step. Feel free to cut kernels from the cob and use raw, or use frozen. Canned is not my favorite, but it would probably be fine as well.
Any firm fish is fine including but not limited to salmon, halibut, snapper, sea bass, rock fish, cod, tilapia. Keep in mind thickness when cooking - about 10 minutes per inch thickness. Prick the bag through the fish, and it should move through easily if it's done. If it meets with any resistance, pop it back in the oven for a couple of minutes.