C.L.
I haven't been there recently so I don't have any specific tips other than to say we have taken our boys 2 or 3 times and we loved it every time. We stayed in Carlsbad, in some sort of Hilton or Marriot property that was family friendly. Maybe a Hampton Inn, Embassy Suites or something like that. There are beaches nearby, but I don't know what they are like at that time of year. I dont' remember the lines being long, but we never went during spring break. We were there in October and May. We never did anything in the evenings other than dinner, swimming in the hotel pool and getting to bed. Have fun--I love Legoland.
ETA: I do remember that the 2 day ticket is a much better deal than one day. I don't remember a 3 day, but maybe you should call and ask. Two days is just about perfect without the Aquarium. The Aquarium wasn't completed yet last time we were there so I can't comment. Also, they do have a significant AAA discount if you have a membership.