Barcelona, being a big city, is utterly unlike the surrounding countryside. The food is fantastic, with a lot of vegetarian options. As with many European cities, eastern influences have actually improved the range of non-meaty food available, and among the many choices there’s a smashing falafel/hummous/halloumi bar convenient from the Sagrada Familia, on the corner of (I think) the Carrers de Mallorca and de Sardenya.
Travel across Barcelona—important for us as we generally have to get off the trenhotel at França station, get to Sants and then head south to my parents—is as good as across London, with a clean, prompt, frequent metro, entirely covered by a single 1,30EUR ticket. Leave França station, turn left and left again and you should quickly reach Barceloneta metro (local map). Two lines later, you’re at Sants.
Alternatively, given the trenhotel arrives at breakfast time, you could do a lot worse than take an hour out to pop into Nakupenda, almost right outside the station on the Avinguida del Marquès de l’Argentera. It’s a smoothie bar, but—before your anti-globalization heart understandably sinks—it’s the way the Spanish would do smoothies. So it’s vegetarian, and fresh, and healthy; but it also does Spanish beer, smooth-yet-strong coffees, and a breakfast deal involving espressos. There’s probably even tapas if you get there at the right time of day. As it’s in the big city, almost any time is a good time: I think it’s open all day.