If you are in the San Sebastian area during Carnival time, I would suggest you take the train 30 minutes down south to Tolosa. The carnival in this city just has a better atmosphere than the one in San Sebastian (unless you are looking for an event that is more calm and you are happy with just watching a parade).
I think that the main reason why the carnival in Tolosa is better than the one in San Sebastian is because it is not just a parade that people stand around and watch. In Tolosa, you are actually interacting with the carnival particpants. You can go up to the people in each float, takes pictures, and if you want (and they want!), you can hang out and party with them for a while. We went to so many floats and talked to tons of people who ended up giving us free food and drinks.
If you like to have fun and are looking for an exciting atmosphere, you will have to go to the Tolosa Carnival. We had a blast there and I think you will too!
Click below for a website that has more details about the actual event.