If you are an RCI member you may have a discount available for the Go Card - there was a coupon code in Fall 2015. Not sure if it's a better price than Costco offers or not.
Our family used the 4/5 day GoCard as much as we could in four days. The Zoo and the Zoo Safari Park (out toward LegoLand) were fantastic. We visited SeaWorld, museums at Balboa Park, the amusement park at the beach, LegoLand, the Midway and a couple of other things. We were pleasantly exhausted. Well worth it.
Of all of those, the beach-side amusement park and the Safari Park were my kids' favorites. You can go to the amusement park EVERY DAY and you get an unlimited ride band, a zipline and mini golf pass, and that thing where you climb on ropes and wear a harness. Balboa Park museums were popular, too.