I am a US citizen. I'm planning to drive from Spain to France with a night stay in Andorra. Will I need a visa for Andorra?
No - no border controls either
Can I ask why you are going there?
I presume the situation will be the same for you as it will be for us Brits post Brexit, namely that to drive in France and Spain, that you will need 2 different IDPs as they are on different systems.
You won’t need a visa for Andorra, but it’s not in the Schengen Zone, so there will be border passport controls
The distance between Spain and France is short, so unless you plan on doing a lot of shopping, or just want to say you spent the night, there’s no reason to spend a night. No visa needed. I didn’t have to stop for passport control; simply drove from France into Spain without stopping, but that was a number of years ago.
We're going to stop in Andorra for lunch only. Sounds as though that ought to be enough time
There is some lovely scenery in and around the country of Andorra, but the capital, Andorra-la-Vella, is now said to resemble an outlet mall (tax-free shopping). I haven't been there recently, but it was no great beauty even back in 1972. I'm sure there are prettier places to stop for lunch, and you might run into some traffic in the town.
If your route happens to take you past Puigcerda or La Seu d'Urgell, I can vouch for their attractiveness. However, I didn't have a car when I visited them, so I cannot comment on the availability of parking.