I was thinking about buying a car or a motorcycle, so I did some research on the rules for American drivers in Spain.
First, in the short term, you should not have a problem. If you’re going on vacation, you just need to rent a car, and most companies accept your license eagerly.
Now to the more complicated process for Americans residing long term in Spain. You can legally rent cars without much trouble, but if you want to buy a used car and use it for a year or so, then you’re going to have to get an international driving permit. AAA offers this and it’s not very hard to get, nor very costly (only 15 bucks). Here is the application, which you can either print out and take to a AAA office in the US, or you can mail it to them if you’re abroad.
In Spain, you can drive for up to one year legally with your international driving permit, after which time you’ll have to get a real Spanish driver’s license.
A final note: there are some websites offering international driver’s licenses. I’m not sure if they are scams or other companies which offer you permits but call them licenses. Either way, you’re better off just dealing with good old Triple A. SpainExpat also says that not all US states’ licenses work abroad (apparently states can make individual agreements with foreign powers) but that the Spanish authorities are unlikely to question any of them if you have your international permit, so it’s not worth worrying about.