I'd look into combining Park Güell and Sagrada Família in one day. They are both a bit out of the centre, in the roughly the same direction, in areas where you will probably have no other reason to be.

You can do (for instance) Park Güell in the morning, and then walk down to Sagrada Família. (And have a look at, or visit, the Recinte Modernista de Sant Pau on the way.)