1Heat gas or charcoal grill. Cut long sheet (about 24x12-inch) of heavy-duty foil. Pile yellow squash in center of foil; top with 3 of the thyme sprigs. Top with zucchini, remaining 3 thyme sprigs, the bell pepper and garlic. Cut butter into 8 pieces; place around vegetables.
2Bring up 2 sides of foil so edges meet. Seal edges, making tight 1/2-inch fold; fold again, allowing space for heat circulation and expansion. Fold other sides to seal.
3Place packet on grill over medium heat. Cover grill; cook 15 to 20 minutes, turning packet over once, until vegetables are crisp-tender. Carefully cut open packet, allowing steam to escape. Transfer vegetables to serving bowl; season with salt and pepper.