Any self-respecting Vallisoletano will be more than glad to take his visitor to sample grilled sepia (cuttlefish) served with garlic mayonnaise in the bar called -- you guessed it -- Bar La Sepia. I used to go to the branch beside the Ayuntamiento, but there's another one in faraway Parquesol. At 7 euros a ración, it's a nice way to start the evening.
Since we're in the area, why not go to the nearby Restaurante Bar El Jero, a haven of delectable tapas (with cool name, btw) that will make you go wild! I personally recommend the shrimp and boletus with peanut brittle and cecina with dulce de membrillo. Be here early because people pack the place to bursting. Closed on Tuesdays.
If you're craving for something heavier, head to Restaurante Los Zagales and sample their delicious Castilian dishes. On a normal day, you can get the entire meal for around 20 euros. It becomes pricier on weekends and holidays. Do order stewed rabo de toro (bull's tail) as second plate and you'll never go wrong.