One thing my son has always been obsessed with was planes. We started with paper planes, mainly the classic squarey one I learnt in school that has good balance of speed/airtime and tolerance to launching speeds and angles.
But he got bored and wanted more. We got deep in the rabbit hole of purely paper folding planes (and rockets), with regular visits to Ojimak[0] for more ambitious projects (they're 3D, glued, yet actually flying paper models).
Our latest endeavours involve keeping large Amazon delivery boxes to later take measurements, calculate weight balance, and creating airfoils by stacking several layers of cardboard in a tapered way to make gliders to throw outside (over 1m wingspan!).
In one of our walks we saw a man trying to put order in his garage; it was literally overflowing with home made RC planes, some were copies of standard designs, some quite unorthodox and some just plain head-scratching weird. We talked for a bit, he didn't even have technical background, and I was sold. Obviously it gets more expensive in terms of time and money, but I can't wait for my son to be old enough to dedicate time together in this direction.
This was magical to me. My "mentor" was able to build tiny butterfly-like contraptions with four flapping wings, and many other flying machines of different kinds.
Maybe this is interesting to your family as well!
As a kid, I was introduced to the concept of ultralight[0] aircraft when me and a couple of friends stumbled upon a wreck of one in a field. Our parents realized it had to have come from the local place a few miles away. If your aircraft qualifies as ultralight, you do not need a license to fly it. A family friend of my parents had one that he'd roll out to the street, attach the wings, and take off, and then land back on the street, remove the wings, and roll it back into his garage.
These things were essentially go-karts with wings.