I spent some time talking with the guy at my dealer that does all of the custom accessories and he recommended a couple of items and told me that a couple of thing were basically pieces of cr*p.
My recommendation, first and foremost, if you live anywhere except Alaska and the North Pole buy the custom fitting sun screen for your smart. The car has a HUGE front window and the screen will make a huge difference in the heat level inside the car when it's parked. Heat and UV are both bad news for a car over time. The sun screen is only $30 from the dealer.
He recommended 1) floor mats and luggage area mat if you plan on carrying anything that is dirty like plants from your local Lowe's. 2) I don't know what it's called but its ~3"sq by the length of the luggage area foam bar that velvros to the floor and keeps stuff from sliding off when you brake hard.
I didn't get the luggage compartment mat as if my wife wants plants or planting soil we can take her corolla. I did by the luggage dam as I don't want things falling off the ledge, it's about $20.
I bought the rubber floor mats because my work requires that I get my shoes dirty and I didn't want to have to clean the carpet ones. Also the rubber ones have a small ridge around the edge that will better coral all the pine needles. The rubber mats from the dealer were only $50 so why spend $30-40 for ill-fitting after market ones.
He also recommend that you either keep low deductible comprehensive insurance on the car with glass coverage or get a windshield wrap. There is some sort of clear material that you can have installed on your windshield that will keep it from being broken by rocks. Its the same stuff they put on sky scrapers to keep the windows from breaking. He told me that they have had four cars already that needed to have their windshields replaced because of rock chips. It about $600 for a new windshield installed. If you look at the front of your smart the windshield is like two thirds of the frontal area and with no hood to deflect rocks the smarts are prone to broken windshields.
I just keep $0 deductible glass insurance so I probably wont get the windshield cover.
Also, either replace the door speakers or make a deal with the parts guy to give you all the take outs. The stock speakers are all the same regardless of model and they are a piece of crap. Not just sound wise but construction wise. It's not a matter of if you will need to replace the stock speakers but when. They are so cheaply built that even the low wattage stock radios will break them.
I was thinking about the arm rest but he talked me out of it. The unit that smart sells has very little structural integrity and will not stand up to being pulled on or having any weight placed on it when you shift in the seat. If you spend most of your time in the smart alone, put the passenger seat down and use it for an arm rest. I do it and it works well enough until I can get a good after market product.
He also half way recommended getting a wrap for the front of the car, especially if you have any color besides black. The panels are make of plastic and he said that while they don't dent they are seeing them come into the shop with divots from flying rocks. Also, only the black panels are the same color all the way through. Evidently, the non black panels are painted and not a solid color.
The wrap really is a matter of how fastidious you are about your car. If a few divots don't bother you then its not worth the money.
Enjoy your new car.