I've felt like I *needed* two days at the zoo, but I have *wanted* two days at Seaworld.
To be happy with one day at the zoo, we planned the MUST sees...and we saw those animals, and the ones in between, and called it a good day. And we were still exhausted.
We ended up with two days at seaworld b/c they often have "buy one day get two days" deals there, so since it was free, we went. And it was great! But it wasn't one visit right after the other, there were a few days in between (my brother lives in SD and it was during a visit to him, NOT a big theme park trip).
Everyone DS talked to (on the plane, at the store, wherever we went) said that
Legoland is PERFECT for a 6 year old....
But on the other hand, DS loved the little bits of time we spent in downtown SD! Walking along the embarcadero (he really wanted to go on/in the "pirate" ship that's actually the maritime museum), then later he and his aunt went to Ghiradhelli and took a pedicab ride...the kid was in heaven! So there's something to be said for down time!