One of my favourite things about German Christmas Markets is the mug system. I don’t think “mug system” is the technical term but it works for me.
When you buy a hot drink at the market, you will pay a small deposit for the mug that the drink comes in. When you are finished with your drink, you can return the mug and you will get your deposit back. If you have decided to go for a second (or third/fourth/fifth) drink, then you can just hand over your mug which will be refilled and you will pay a little less since you already have the mug.
However, if you become really attached to your mug, even when it is empty, then that is no problem. You are free to keep it without an ounce of guilt. Even better is that each town has a different mug and they usually have the name of the town on them, making them a perfect reminder from your trip!
During the Christmas Market trip, we kept one mug from each town (on one or two occasions we actually kept two). Below you can see all the different mugs that we came across.