Barcelona is quite spread out. I'd say best to concentrate on one neighbourhood and get in 2-4 hours of quality time there, than running around seeing a probably closed Gaudi Catherdral.
I'd recommend finding some food in the el Rival district. There should be something open there. It is bit touristy. There are paella offerings on almost every restaurant, but you should be able to find something there. You can wander the streets and alleys until you get tired then crash out at your hotel.