Prepare your camp stove or campfire. For a stove, set it up on a stable, level surface away from tent walls and dry leaves. For a fire, let it burn down to glowing coals with no active flames.
Tear two large sheets of heavy-duty aluminum foil, each about 18 inches long. Lay them flat on a cutting board or picnic table with the dull side facing up.
Brush the center of each foil sheet with olive oil, leaving a 2-inch border dry.
Place one mahi mahi fillet in the center of each oiled area. Drizzle the remaining olive oil over the fillets, then season evenly with salt, pepper, and garlic powder.
Top each fillet with lemon slices, thyme sprigs, and butter pieces. Add cherry tomato halves if using.
Fold the foil tightly: bring the long edges together and fold over three times to seal, then crimp the short ends. You want a packet that's sealed but has a little air space around the fish.
Place the packets on the camp stove grate or directly on the coals. Cook for 6 minutes, then carefully flip with tongs. Cook for another 6–8 minutes, until the fish flakes easily with a fork. Thicker fillets may need 2 extra minutes.
Remove from heat with tongs. Let rest for 2 minutes. Open the packet away from your face to release steam. Serve straight from the foil or slide onto a plate.