Airplanes! Other than the food, what's not to love? Source of wonder for children and adults alike, the airplane is a relatively new addition to our world, coming into being in 1903. Orville Redenbacher...ahem, that's Orville and Wilbur Wright, designed, built and flew the first airplane in Kitty Hawk, North Carolina.
As with most technology that can be applied to wartime uses, the art was quickly advanced through both World Wars. In just forty years, airplanes advanced from handmade gliders with an engine strapped on to jet-powered behemoths.
While military and intelligence interests pushed the state of the art after World War II, so too did the commercial airline business. Jet power, metal airframes, and pressurized cabins allowed aircraft to soar higher and fly faster than ever before. With the Concorde, airline passengers could fly faster than the speed of sound!
Nowadays, large passenger jets are focusing more and more on fuel economy and weight savings. Such areas of emphasis have given birth to aircraft like the Boeing 787 Dreamliner, the first jet airline to use mostly composite materials (in this case, lighter, stronger, and generally more expensive than metal).
Wow, there are just so many activities for airplanes. There's the always popular paper airplane. You can even go to a local airport and watch the planes. When your kids are old enough, maybe they can play "flight attendant" and hand out peanuts and absurdly small bottles of water. The best activity (for the older kids) is the Experimental Aircraft Association's Young Eagles program. Children between the ages of 8 and 17 are welcome to participate. A volunteer member of the EAA will teach the child about the airplane on the ground and then take you flying for about 20 minutes! I was lucky enough to have this experience when I was 15. A local pilot took me up in a Piper Cherokee and even let me fly the plane for a little while! It was a wonderful, memorable, and free experience. It could be the start of a new hobby or even career aspirations for your child. While I didn't become a pilot, I maintain a hobby flying remote control aircraft and became a mechanical engineer with a concentration in aerospace.