Okay, I have two "necessary" mods I have made: louder horn (WOLO BadBoy to replace the dinky "excuse me" horn that is stock), and cruise control (for long distance driving. Area 451 cruise).
I have a number of others I have done - both mods and add-ons: Interior trim paint, Coco floor mats, shoulder strap pads (could be considered "necessary"), "truck" lights, some decorative stuff (see my gallery), tow hitch, "periscope" lens (for seeing traffic lights when first in line), trunk organizers, etc. with more to come.
The great thing about the smart - it lends itself very well to customizations, both inexpensive and expensive, or is perfectly fine the way it is. Only you can be the judge of what becomes necessary, though I would strongly recommend floor mats of some sort - the stock carpet isn't all that robust. Whatever turns you on (to borrow from the Caddy ad) so that your car can return the favor...