In the US and Canada smart cars are not rated for towing. In europe the gas smarts with 1000cc are rated to 440 kilograms and the diesels are rated to 300 kilograms (both plus or minus) I have JPEGS of the German stickers but think they are on the old laptop.
Uhaul has the hitch which is made by Curt. Seems to me they `are just under 200 bucks. Wiring is easy and is just split off the tail lights. 20 minutes. Although less common on this side of the pond there are lots of smarts towing small trailers. In europe it is more common.
I recommend you look at something like a timeout trailer. We use this setup for weekends close to home. Loaded with Kayak and fridge freezer packed it comes in around 550 pounds. Our smarts are diesels so we have to watch the weight. Having said that it pulls a 6 percent grade with no problem in 4th gear. Braking is excellent and it is extremely responsive on the road. And having said all that we seldom go more than a couple hours from home. Local lakes etc. There are a few pics below from various places we have gone. Setup is 10 minutes...quicker if it is raining. :)
What you see holding up the Kayak is actually the legs of the bed end of the trailer. Very slick.
Two of our smarts are rigged for towing.









We can also pull the smart cars own trailer if we have to although I don't like pulling this trailer any more than I have to. Good enough to move it around a campground etc.

Feel free to ask any questions. Personally I would never tow more than 700 pounds with a gas smart and keep the hitch weight no more than 85 pounds. Our hitch weight is 65 pounds.