I took the kids to the Stadtfest in Ostheim last weekend. The absolute favorite attraction for the two older boys is the bungee trampoline jump. They totally love it. It's expensive (5 Euros for 5 minutes) but oh, the sheer joy. For some weird reason, Alan always (for years now) shouts, "Geronimo!" at the top of his lungs and laughs, laughs, and laughs. Hard to resist that.
Also, yucky slush drinks. They are horrible, they turn your kids into colored monsters with blue teeth but oh, the delight.
Ice cream, fire-popped popcorn, plastic microscopes from the flea market, boxes full of train tracks that Opa buys for Jacob... thus are childhood memories to last a lifetime.