The Pyrenees is the mountain range that divides Spain and France. Nestled amongst those mountains is Andorra.
Practical Considerations: Currency and Border Controls
Andorra is not in the European Union. However, the country does use the euro as its currency, the same as Spain and France.
Andorra has border controls between itself and both Spain and France. Usually crossing the border is quick and easy, but you can never rule out delays.
How to Get to Andorra from Barcelona
There are no trains to Andorra, so you'll need to come by road. Be careful when using search engines for working out your routes and times, as there is also an Andorra in Spain. It will usually be called 'Andorra, Teruel' on these sites.
By Bus The journey takes between three-and-a-quarter hours and four hours by bus, with the ALSA Bus Company.
By Car It takes around two-and-three-quarter hours to get from Barcelona to Andorra by car, traveling mainly on the C-16 road. Note that there are tolls on this road.
Andorra by Guided Tour
A very popular tour is the Three Countries in One Day - Spain, France and Andorra tour from Barcelona, which takes in the French town of Mont-Louis, the Spanish village of Baga and some time in Andorra itself. It would be very difficult to fit so much into your day under your own steam (and impossible by public transport.
But if it's just mountains you want and you're so desperate to make it all the way to Andorra, there are also tours of the Pyrenees to consider.
How to Get to Andorra from Lleida and Girona
Lleida is slightly closer to Andorra than Barcelona is, so the buses are a little quicker: some make the journey in just two hours and 25 minutes, but most take around three hours.
Again, book from ALSA.
There are no buses from Andorra to Girona.
What to See in Andorra
Andorra is actually a separate country (the only country in the world where Catalan is the first language, although Spanish & French are widely spoken). Excellent for skiing and buying cheap electrical goods and jewelry due to the tax-free status of the country.
There are several points of interest outside of Andorra too. The mountain town of Vic, the stone-built village of Queralbs and of course all the lovely scenery.
To see all of this in a day would be quite an effort if you were to organize it all yourself so you may want to consider booking one of the above tours.