Best season: unless you are a ski fan, I enjoy it better in August, hotel rooms are cheaper, coaches are rather empty, and they have a night show in Caldea.
Distance: 3 hours from Barcelona, regular daily coaches.
3 hours from Sants Estació (Metro L3), come out and find the coaches section.
Eurolines has a counter there, you can buy online too. (beware of pickpocketers in Barcelona's coach station).
You must check in at the counter before travelling.
Andorra is a little country between Spain and France, completely independent and tax free, coach is not the cheapest of options but I do enjoy the scenery (and it can be quite a long day ahead).
The streets are full of shops with perfumes, watches, sunglasses, tobacco, biker's gear, chocolate, etc.
The great thing about going in the low season is that hotel rooms are pretty cheap, I paid in Summer 2013 46 euros for a night, 4 star hotel for 2 people. Bargain!
Map navigation guide here