I also recommend Barcelona and Monserrat. Why waste time making train connections?
Besides the big or famous sites at Monserrat there is also a lovely small art museum up there.
Plenty to see in Barcelona but I'm going to tell you two places not to visit until you have seen more than a couple of days of the other sights.
The first is
Palau Güell, a home Gaudí designed for a patron and, in my opinion, the least interesting of his Barcelona works. The second is Gaudí's home located in
Parc Güell which doesn't show much of Gaudí's architecture. The park itself is worth a visit though. And both sights I mentioned is worth a trip -- someday.
Whatever you want to see in Barcelona go online and buy advance tickets.