Long-handled utensils—especially tongs and a wide-bladed spatula—for positioning and turning a variety of foods.
A fireproof basting brush for oiling the grid (the grill's metal rack).
A stiff wire brush for cleaning the grid.
A cheap aluminum foil pan to use as a drip pan when roasting or, upside down, to partially cover foods that need concentrated heat to cook through.
A powerful flashlight so you can see the food if you're cooking after dark.
A chimney starter to ignite your coals for a kettle grill. It also allows you to light a second batch of coals to add later, for long-term roasting.