Flea market

Take two kids, give them some money (about twenty euros, not enough to cause major damage but plenty enough to give them the sense of responsibility), give them three hours of time and place them in a huge flea market, as the one in Porte de Clignancourt. Then step back, somewhere between 5 feet, where they feel free and independent, and 10 feet, where you start freaking out because you see kidnappers and sexual harassers everywhere) and observe.
They'll stop at every little stand observing and touching and bargaining on every possible object (I was surprised to find out that they noticed furniture and paintings and art objects other than toys and clothing)!

Now, are you ready? Whatever you guessed they would buy (a toy, a fancy hat, a tool...), there's no way your imagination can catch up with theirs.
I'm almost sure I wouldn't come home with a pilot helmet (Giovanni) or a porcelain dog (Gabe).