Extra burner: Ideal for side dishes. Some grills come with one, but be sure there's enough space to fit your pots. A separate burner is about $500, plus installation. Plug-in induction ones that stow away when not in use start at $200.
Refrigerator: A stainless-steel undercounter unit keeps cold drinks and ingredients within reach of the grill. Tack on $600 to the cost of a $300 fridge for a new in-ground electrical line to power it.