I fell short of suggesting missing Park Guell. Thought I might be admonished! It was interesting but not my favourite. As a second Gaudi 'fix' to Sagrada Familia, we thought Casa Batlló was a lot more fun and easier to get to. To us, the park felt like a make-believe land (without a guide). The Gothic Quarter felt real, so well worth the time spent there. I've done the Runner Bean tour twice and hope to do it again this fall. A little different each time with a different guide.
I only suggest a route on the HOHO for a relatively quick overview, don't get off, time to sit and relax, learn from the English commentary.
Food isn't a priority for us on a port day. For a quick snack at a café, try patatas bravas and a local drink. (BTW, don't lay your phone or camera down on a table. Don't fret. Just keep your bag in your lap.)
Come back and tell us how your day went!