Yup. Wine and champagne corks!
We have saved hundreds of them over the 20 years we've lived in our house (No, we're not drunks). After every party or celebration, the corks went into a wooden container on the stair landing.

Halo and Pepper love to fish them out and bring them to you (usually at 3:00 in the morning) hoping for a game of fetch. We find them all over the house. They never chew them up but just bat them around. The especially love the champagne corks because their shape makes for an unpredictable bounce.