In a large bowl, combine the squash, tomatoes, red onion, green beans, lemon juice, oregano, oil, ¼ teaspoon salt and ¼ teaspoon pepper.
To make a packet, lay two 20x12-inch sheets of foil on top of each other (double layers will protect contents of the packet from burning). Spray the top of the foil with nonstick spray. Place one portion of tilapia in the center of the foil. Sprinkle with salt and pepper; then top with about ¾ cup of the vegetable mixture. Sprinkle a few capers on top and olives too, if using.
Bring the short ends of the foil together and seal, leaving enough room for steam to gather and cook the food. Pinch the seams together along the sides too. Repeat with the remaining foil, fish and vegetables.
Grill the packets until the fish is cooked through and the vegetables are tender, 5 to 10 minutes total. To serve, carefully open both ends of the packets and allow the steam to escape. Use a spatula to slide the contents onto plates.
Notes
The nice thing about making these in individual grilling packets is that each family member can put what they'd like into each packet. Try adding other vegetables like zucchini, asparagus, bell pepper, broccoli, etc.
Other mild, white fish may be substituted for the tilapia.