I'm from Oahu but have some knowledge of Maui.  Maui was a training ground during WWII, but it doesn't have much to look at now.  I did some training at the National Guard armory at Puunene, middle of the island, where there's a couple of WWII concrete buildings and the airfield which is now a racetrack.  I don't think they have a WWII related museum.  You can read more about it:  https://www.tourmaui.com/world-war-2-history/

Maui is known for a large ancient Hawaii battle at Iao Valley where the armies of the Maui King and King Kamehameha fought for control of the island, where the river ran red with blood.  I highly suggest doing the short hike.    https://dlnr.hawaii.gov/dsp/parks/maui/iao-valley-state-monument/

There's a few small gun shops, but nothing to go out of the way for.  The largest is Maui Guns and Ammo.    http://www.mauigun.com/