Atlanta · Georgia
If you're into aviation or have a few spare hours in Atlanta, the Delta Flight Museum is a solid, low-key stop. Expect a decent collection of retired planes you can walk through, old uniforms, memorabilia from the airline's early days, and some interactive exhibits about how flights work. It's not overwhelming in size so you can comfortably see everything in 90-120 minutes. The hangar-like setting feels a bit industrial but gives you real access to the aircraft.
Best time to go is spring or fall when Atlanta isn't scorching or freezing. Midweek mornings tend to be quieter. Expect to pay around $20-30 per adult; kids and military usually get discounts. Parking is free if you drive, and it's only about 10-15 minutes from the airport if you're on a long layover.
Tip: Definitely go inside the planes and the flight simulator area if it's open - that's the most fun part. You can safely skip the small gift shop unless you need a souvenir. Bring water because the hangar can get warm, and wear comfortable shoes since there's a fair amount of walking on concrete.