The aquarium is fun and you could easily do it in 2 hrs. Last time we went was kind of a bummer though because they had a big fish kill due to mislabeled chemicals from one of their suppliers and they were still trying to get exhibits set back up. That had to be 5 or so years ago so I'm sure they've long since recovered from it.
The Lexington is also cool but probably not for kids unless your kids like WWII history. We did an overnight camp with the cub scouts but it was in late October and the weather for it was *perfect*. The kids were bored but all of us dads loved crawling all over that ship. We also got a nice high because they were in the middle of restoring one of the planes and they were painting it the weekend we were there. The resto shop is on the lower deck so the whole ship filled up with paint fumes that night.
Whoever said the Whataburger on the bay is spot on, depending on the age of your kids. Mine are young so they were flipping out about getting to ride an elevator in a Whataburger.