Pee-Wee's foil ball
Occasionally during the first season, someone would give Pee-wee a piece of aluminum foil, at which point he would exclaim, "Let's see how big my foil ball is today!" The ball was rolled out, and Pee-wee attached the new piece in sheer delight. Pee-wee stopped using the foil ball after the first season is over, citing the fact that it became too big to keep in the playhouse. He donated it to the foil museum. It was replaced by a ball comprised almost entirely of rubber bands in the second season; the gag was continued by someone occasionally giving Pee-wee a rubber band (at which point he would exclaim "Wow! A rubber band for my rubber band ball!" before putting it on).
The final appearance of the rubber band ball is in Pajama Party.