Pot Lid "Mouse" Trap
There are some days when it doesn't take anything parent-led, packaged, or planned to entertain your kids (of course, there are also days when it does!). When the stars align and you can sit back and watch your kids make something magical out of almost nothing at all, it's a pretty fun thing to behold. Case in point:
My nephew absconded with this pot lid one evening when I was making dinner. He shortly came back, requesting string because he needed to make a "mousetrap." Just by tying two pieces of string around the handle of the pot lid, this is what he came up with.
How We Did It
With my help, he tied one of the strings to our baby gate, and then he gave the end of the other string to his delighted little brother. Together they happily trapped various toys by raising the lid, sticking the toy under it, and dropping the lid. I had to confiscate it in order to get them to get ready for bed.
This is, of course, the two-man version of this activity, but kids without an accomplice in the house could easily make a more portable one-string model.