Right, I might add some knowledge I gained about German SIM cards this year. A bit late, but it's something that can be useful for next year. Maybe someone can make a page about the known info in a more gathered form.
If you're a foreigner - don't get a "Lidl Mobile Smart" SIM card, as that will require you to have a German bank account. This information was or is not (clearly) informed about on the package itself.
I ended up getting a o.tel.o prepaid SIM card. The card itself costs 4.99€ and comes pre-charged with 5€. However, to get "Flatrate Internet", one'll have to top the card up with an additional 4.90€, as that package cost 9.90€. Although, top-ups are available in sizes of 9, 19 and 29€, so the total cost will be 13.99€. O.tel.o is a Vodafone brand, so it'll use Vodafone's network. The card need to be registered/activated prior to usage, which can be done online. I do not know about the validity time of the card, though, but my other Vodafone SIM card is still valid, just in a somewhat weird limbo state, so didn't bother bringing that one this year.