What are good places for a day trip from Barcelona, Spain.?

Without a doubt you must see the monastery at Montserratt. It's about 50 kilometres north of Barcelona. I was there in May and it is an awesome place high up in a mountain. You can drive up, take a cable car or do what I did, take the funicular. Make sure you have plenty of film in your camera. It is a super day out. Enjoy.

Edit: About 40 kilometres north of Montserratt is a delightful town called Cardona. There is a Parador (hotel) there in a 8th century castle. It is well worth a visit if for only a drink on the terrcae. I have been privileged to stay there and the experience was awesome.

In addition to the Monastery of Montserrat, with a scary telespheric ride, you can take day trips to the small country of Andorra or the city of Tarragona.
Andorra la Vieja is a beautiful small city in the Pyrenees Mountains between Spain and France. The place is famous for shopping and hiking.
The historic city of Tarragona, founded by the Romans, have a nice city center with a Roman amphitheater. If you like the amusement parks you can find a good one there: Port Aventura.

Girona is pretty interesting

Both Girona and Tarragona are great day trips from Barcelona.. I think if you have the time , visit both.. They are both 1 hour away or so from the city .....

Take care..

Bcn_mimosa from Barcelona, Spain

Another nice visit is Sant Sadurni d'Anoia where all the wineries that make Cava (spanish equivalent to french champagne) are located.There are several that have guided tours, such as Frexenet and Codorniu... I've done the Codorniu tour of the bodega, and it's very interesting.
If you enjoy beaches take a trip to Sitges, very small with a great view of the sea, lovely beaches, and great seafood! Or you can go north past Girona to Figueres and visit the Dal铆 museum. Or south to Castelldefells. Also a pretty little city with nice beaches... There are quite a few to choose from!
Whichever you pick, have a good visit!
:-)