I’ve already visited all of them except Monaco, but not all in one go, just gradually across the years. Andorra I went skiing in, Vatican was a side quest in Rome, Luxembourg I just went for one night on a whim, Liechtenstein was a side quest from Zurich, and San Marino was a side quest from Bologna.

I may be an adult on the outside, but…

I took the bus up to San Marino when I was nearby once. There was a sign near the bus stop about having one's passport stamped, 5€ at the tourist information office and by chance I had my passport on me. Happy happy joy joy.

(Lots of businesses use .sm domains, yes.)

I've wanted to do exactly this for many many years.