I'm an uncle who gets to see my nieces and nephews rarely. I just got back from a trip where we were clearing out grandma's house; she's a hoarder and kept a bunch of her daughter's toys, so 3-yo grand-daughter had a sudden cornucopia of age-appropriate new (to her) toys to play with.
One day she ended up in my wife's and my room, helping us pack for the return trip. She latched on to a small empty travel pill bottle and started zooming around the bed with it like it was a car (although, sometimes it flew). The bottle looked nothing like anything; just a random short cylinder of plastic, but she amused herself for at least a half hour with it.
Boundless energy; boundless imagination. We're quite amazing creatures.