El Estadio Nuevo Los Carmenes is a bit old fashioned inside, with the facilities somewhat lacking compared to other La Liga clubs, meaning there's no real "best” food and drink here. I'd recommend getting toward the stadium fairly early and getting a drink in one of the bars neighbouring the venue, such as the Bar el Estadio, where you can also get some food. Taxis from town are quite cheap (€8), so you could even drink in your preferred bar there before heading in.

During your stay in the city, you will see it's impossible to be hungry. Every time you order a drink in any bar, you will have an interesting amount of food with that. You can find snacks and drinks inside the stadium but food is better if you go somewhere outside. There are several bars around the venue like El Penalti or El Estadio. Cheap and nice tapas and friendly atmosphere. You don't need anything else!