I once found a wallet here in Japan full with money and cards too. It was Saturday and I took it to the closest Koban (tiny local police station, but always open) and handed it in.
The made me wait while the were going trough it and documented every piece and card. It took me about 90 minutes before I could leave. It felt like a punishment for me for being nice but giving them something to do.

Next time I will just drop it into the mail box there or similar, to avoid the hustle.