The Parc Guell is well worth the trip up an incredibly large hill! It was one of the highlights of our time in Barcelona and we literally could have spent hours there, people watching, looking at the beautiful Gaudi architecture and mosaics.
A suggestion would be to bring your own lunch, as the food options are limited and expensive (although the simple sandwiches we had were tasty). We actually came in the back entrance to the park, so we didn't see the yummy tapas places that line the street leading up to the main entrance. A must see for all Gaudi and architecture lovers!