The aviation exhibit is located in Treacherous Mansion and also one of the only rooms which is totally optional to enter along with the Archives in the regular mission, there its a room filled with planes, balloons to decorate it, making the mood like the sky, there's a balloon with 2 sand bags. In E-3 " A train to catch ", if Luigi wanted to get the star gem, he should pull one of the sand bags on the balloon, but first, after pulling 1, some Strong Greenies and Strong Hiders will attack and Luigi will be unable to pull the 2nd sand bag until they're defeated. Then, Luigi will claim his star gem. In E-5 "Paranormal Chaos", Maraboo is also in there, Luigi should use the dark light to find the missing balloon then it'll appear.