We loved Barcelona and support Zacs recommendations, especially the Gaudi sites. Park Guell is a beautiful place and needs best part of a day, the beach is worth a stroll if you have time and want some peace and quiet away from the crowds, followed by a cable car ride up to Montjuic.
The fountains and the outdoor escalators down from Montjuic at te north end of the park are a must.
(Despite being Mr diligent, my husband lost his wallet too. He had it pickpocketed when he gave way to two teenagers as he was getting on the subway train. Luckily our passports were in the hotel safe, but he lost cards and cash. Take a small amount of cash out with you each day and no unecessary cards. Share the cash and cards between you in a purse under your clothes or zipped inside pocket.
Do not be afraid, it is a safe place otherwise, just pestered by teenage young professional pickpockets on the trains, at stations and in te crowded places.
I'd go back at the drop of a hat.