Held every year on “Easter Monday” (the day after Easter), people gather in the old-town vicinity of Rüdenplatz to play this game. As noted by the city’s tourism web site, “adults throw 20-cent coins at hard-boiled eggs held out by children with the aim of making them lodge in the shell. If they succeed, the thrower keeps the egg and the coin. An almost impossible task – much to the delight of the children, as every coin that falls to the ground instead of sticking in the egg belongs to them.”
I have no photos (of my own) to share – but that tourism page (now gone) has a few brief video clips, and an Aargau newspaper posted a cute video a couple years ago (now removed). Wikipedia has a few more details [in German] about this fun tradition related to “egg tapping” practiced elsewhere.