Well, buy a new backpack and take pictures of the autumn colors tomorrow :)

You're not moving tomorrow, so plenty of time to make packages to send home yet and write lists about what you need to do and who you need to inform. You're there for a few more months, so you can worry about the Arbeitszeugnis until later. Why not make it in English, your boss should understand and your future company too :)

You can ask Mandarake to pick up your stuff in case you don't want to take it with you later too, why worry about it now?

And about your sales: It's the weekend! Slow down. I wouldn't spend a beautiful day at home just to sell something. They'll live if you reply a day later.

Prioritize! What can you only do now? Yup, autumn colors!
Prioritize! What of these things do you need done now? Yup, buy a backpack as you need it every day.

Everything else, do later.