Hi there,
We stayed in the Princess hotel right at the bottom of diagonal last year for a couple of days and really enjoyed it. It has a pool/bar area on the first floor, and a rooftop lengths pool also. The room was lovely and we had the kids in an adjoining room (with a connecting door). The glass wall to the bathroom was a nice novelty! We found a good deal online. Only thing is it is a bit far from the ramblas/center etc, but there is a large shopping center with plenty of restaurants right beside it, and the transport links are good.
My sister also stayed in the hotel marina? at the bottom of the ramblas and that was really lovely. Beautiful contemporary rooms and very close to the action with the ramblas, barceloneta and maremagnum (shops, cinemas, aquarium) almost next door.
I lived in barcelona for a year, so if you have any more questions, ask away! It's a fantastic city.