Are you a member of Southwest Air's Rapid Rewards (frequent flyer)? It's free to join and you don't need to actually fly with them to be a member. Anyway, with the RR number you get 20% off a full-price admission.
If you're thinking you might spend two days there, they sometimes have buy 1 day get 2 days directly from their website. My brother lives in San Diego so we knew we'll be back at least twice in a year, and in December they had a "buy 1 day get a year" passes, so yeah, we got those.
If you're interested in that, I do believe today is the last day to buy those from their website...not sure when you have to activate them, though, and there are approx 5 days this year you can't use them (listed on their site).
I think you need to go to the site,choose San Diego, then click on either the passport link or the multi-day ticket link.