First of all, you are in for the most amazing time. You have to do pintxos for dinner. It is where you travel from restaurant to restaurant having one small dish and a glass of wine at each place for your dinner. Go to the street 31 de agosto in the party vieja (downtown area) and try this pintxos path to start. 1. Gandarias and order the solo mio (incredible bite of steak on a piece of bread and green pepper with salt on top) 2. A few doors down, La sepa. Get the mushroom and bacon dish. 3. Fuegro Negro. Most interesting place. The Kobe burger is a must, the risotto is incredible and the dove ain't bad either.
For a nice sit-down dinner, go to la cueva, also on 31 de agosto. AMAZING mushrooms and shrimp and the family-owned and operated place has an great atmosphere.
Other than food (it is the food capital of the area), a walk up to Jesus (you'll see him) is a must, the zurriola beach is less famous but is the surfing crowd, the aquarium is really neat and walking around to the other side of la contxa is an iconic view. Also take the tram up to mont iquell and see the park.
Also.... Pagadi bar in barrio gross has THE BEST papas bravas in Spain. Hands down. about 6 years ago
You must go for pinxos at the little restaurant that is off of the main restaurant row with the spoon on the sign. Also the price fix lunch at La Perla is fantastic. 18 euros during the week ant it is right on the main beach. I just got back from there yesterday. about 6 years ago