I gathered it all up, and sorted it out on my desk at work. I found a charge return receipt with a phone number on it. I called the number, and reached the owner of the wallet. He was in North Seattle (I'm in Kirkland, a half hour drive away), and had just noticed it was missing, and was in the process of cancelling his cards.
He was sad and disappointed that there were none of his IDs or his photos in the stuff I found, but was glad and touched that I would care enough to call him. I got his address, put it all in an envelope, and dropped it in the post.
Lost things make me sad. In fact, losing things is sadness, almost axiomatically. And thieves make me angry, almost beyond reason.