I stayed up until 12:30 AM last night researching Andorra, which I tentatively plan to visit on Tuesday, Dec 28 as a day trip from Barcelona if COVID and other circumstances permit.
Near far as I can tell, the vast majority of tourists who visit Andorra do so as part of a standard "3 countries in 1 day" tour that every single tour company in Barcelona offers. These tours all depart Barcelona about 7:00 AM, stop at the small town of Baga in Spain, then stop at the small town of Ax-Les-Thermes in France for lunch, then enter Andorra from the north (where there is spectacular mountain scenery) before stopping in the Andorran capital city of Andorra la Vella. You get about 1 hour in each of those places and in total it works out to 9 hours of sitting in a bus and 3 hours of being outside of the bus visiting places.
I have booked such a tour, fully refundable of course: but I wondered if I could do better myself. And I believe I can.
The Spanish bus company ALSA runs an express bus between Barcelona and Andorra la Vella each day. The bus to Andorra departs at 7:00 AM and takes 3-ish hours to get there. The bus back to Barcelona departs at 7:15 PM and likewise takes 3-ish hours to get back to Barcelona. That would leave me with about 6.5 hours in Andorra.
Andorra has a surprisingly robust system of public transit buses that connect Andorra la Vella to surrounding villages. There's a town about a 20-25 minute walk south of Andorra la Vella called Santa Coloma that has a church from the 9th century that I'd like to see, as well as a historic old bridge (one thing I love about microstates is that their major cities are often within walking distance of each other!). A 15 minute drive north of Andorra la Vella is a charming small town called Ordino that has cobblestone streets and a number of old houses you can tour. I think I can fit both of these charming small towns into an itinerary that fits within 6.5 hours, though it may require some bus luck.
Most people visiting Andorra in the winter who are not just day trippers from Barcelona go there for the ski resort, which is also a bus ride away from the capital. But I was evacuated from the slopes last time I tried to go skiing so that whole side of the country is not for me.