I’d get down to La Barcelonetta area, and just pick a restaurant you like the look of. Close to the harbour, so the seafood is always good.
This.
The main tourist areas of the City are generally more expensive, with very tightly packed sitting, not quite as good service (staff more concerned with dragging you in rather than serving food) and less nice food.
The Harbour area is quieter, cheaper and the food is amazing.
I can’t think of any particular names, there a place right down the on harbour I like for lunch, it’s got a huge mirrored wall above it, despite what I said about it’s one of the most touristy places in the city, but it’s the best place to watch the world go buy and drink lovely cold beer and have seafood tapas. I wouldn’t go there in the evening though.