When you want a grocery cart you have to put in a coin to release it from the chain of the other cart. You slide it either in the top or the front, depending on the cart. When you return the cart you slide the key back in the slot to get your money back. Do this whole thing before you go in the store since you can't get out of the store unless you go through check-out which is always busy.
When you order a beer at festivals it will be served in a real glass. They will charge you for the glass. You then return the glass when you finished to get your money back. It's call a Pfand. Usually it's 1 euro but German litre glasses are usually 2 euro. The glasses at the Christmas market were 1 euro and were so cute I kept many of them. That is acceptable. Wine glasses at the wine fests are all unique and fun to collect too. I paid a Pfand for a plate once also.