From Tom ("devoted Parenthacker and Jersey exile"):
While standing in line at the grocery store the other day I stumbled upon a Parent Hack I'm rather proud of. My daughter is almost 4 and very fond of playing Memory, so when we got stuck on a fairly busy Sunday line waiting for checkout I took a handful of the new quarters and sorted them into pairs by State (you can also have a pair of the normal "eagles" and the 1976 Bicentennial quarters as well). Turning them heads-up so they all looked alike, I spread them out on my palm and we played Memory until it was our turn in line.
I was surprised at how quickly the time flew by – we couldn't have played more than two or three full games, just enough for it to keep my daughter's interest!
Related: Pocketmod for printable, portable entertainment