As far as hooky family movie conceits go, the "Night at the Museum" franchise is centered around a pretty adorable one (one that, it should be noted, is based on a 32-page children's book of the same name by Milan Trenc): thanks to an enchanted tablet, every night after dark, the exhibits at the American Museum of Natural History come to life. A night watchman, played by Ben Stiller, finds out the secret and has to keep all of the museum's inhabitants inside the museum. For a single movie, even under the artless direction of Shawn Levy, the idea played well, especially with a supporting cast stocked with comedic heavyweights like Robin Williams, Owen Wilson, Steve Coogan, Ricky Gervais and Dick Van Dyke. With the original film, there was enough genuine whimsy to sustain, but now, with the third film, "Night at the Museum: Secret of the Tomb," even...