While not a "must-have", I would also add something for storage and preparation for simple comfort foods. This might be as simple as plates and utensils or as complex as a small fridge, microwave and hotplate/small stove. While going on walks is awesome, having to always leave the office to satisfy hunger/thirst can be a productivity killer, especially if you don't have a corner store and quick eats right nearby.

boom! done :)