We got the diaper bag back! There was a knock on the door this afternoon and a complete stranger was there holding our bag. She said her kids had found it at the park near our house. Most of the stuff was still there, including her jeans, but her books and a couple of toys are gone. Someone had taken the time to dump a handful of sand into the bottom, but that can be cleaned up.
The woman who returned it said that she saw it was full of baby stuff, and our cheque book. She said that once she saw the cheque book she knew we needed it back. So we have the cheques back, completely undisturbed. It weird because everything else was wet from being outside for a week, but the cheque book is fine.