Go to your auto insurance company and just add liabilty insurance for it. I think liability is minimum for all states.
I know in Texas that liability is all you need to register a vehicle. But on the same note, in some cities you need to have a current inspection sticker showing in order to park it on the street, driveway, or yard. Otherwise you have to park it in the back yard behind a concealing fence.
Forgot to mention that parking in a garage with the door closed is permissible without a registration or inspection. IE, it's OK as long as it can't be seen from the street.