I guess the one-day trip referred to before was to Montserrat and not to Montjuic, as Montjuic is in the city and it does not take 90 minutes to get there. Montserrat is actually a very nice one-day trip to take and the mountain itself is just magic. I think it is really worth it to go there. If you prefer the beach I might suggest you to visit Sitges, which is 30 minutes down the coast by train. Girona is also a very nice city and it takes around 90 minutes by train to get there. Whatever you decide to do, I hope you like Barcelona, my home town