[villagetinker]

IMHO,
Seat Belts
Self-canceling turn signals
Depending on your physical size, alternate seats
Additional side lights are nice for visibility
Additional front turn signal lights (the factory ones and are very weak and hard to see)
If you get the optional fold down rear facing seat, get additional brake/turn signals as the normal ones are blocked when the floor is raised. Also consider the extended roof for the rear passengers.
A GPS based speedometer with odometer function is very nice.
A fuel gauge is also very nice.
Side curtains (an enclosure) is needed for foul weather and colder temperatures.

There are other items, but these can easily be added later.