There are certain things, from most places in the world, that make a lasting impression on you, sort of sticking in your memory permanently after you leave.
And there are a lot of things that Germany has, historically, been very good at: philosophy, theater, art, literature, nightlife, and so on; and if you're going there as a tourist, you're probably planning on working several of those into your agenda. But anyone who's ever spent some time in Deutschland can tell you that uniquely German junk food is, without a doubt, the 8th wonder of the world. The under-par food you buy at the späti (convenience store) will live in your heart in a way that no monument or breathtaking landscape can. Don't even ask why. You'll understand when you try it, and you'll miss it when you're back home.
It seems random, but paprika (German for "pepper," as in bell pepper) is the most common potato chip flavor in Germany. They're actually incredible in a way you probably didn't know potato chips could be: salty, a little sweet, and a flavor that has moved people to write poetry (seriously, I know someone who wrote a poem about paprika chips). After a certain hour, it's difficult to pass through an airport terminal, U-Bahn station kiosk, or corner store without grabbing a bag.
Club-Mate, a carbonated drink made from mate extract, is definitely an acquired taste. It's an unfamiliar flavor (doesn't really even taste like mate tea) and not very sweet (which comes with the benefit of being low in sugar and calories--this is why Germans can eat so much junk and stay thin). Once it starts to grow on you, though, it's a great way to get through class/work or Berlin's all-night party scene, as it's highly caffeinated.
Ritter Sport Chocolate
Yes, you can buy Ritter Sport pretty much everywhere in the world, but look: in Germany, it's one euro for a bar. It's just really good chocolate (my favorite is the plain dark, or "halbbitter"), but it also comes with hazelnut, cornflake, strawberry, or various other ingredients. If you're in Berlin, you can visit the store and concoct your own mixture.
The classic Fritz-Kola is basically just a very good cola with a ton of caffeine and far fewer weird chemical ingredients than Coke or Pepsi (because again, Germans are somehow healthier than us even when making unhealthy foods). But the other varieties are really good too, like the melon Fritz-Limo flavor or, inexplicably, the coffee flavor. Sounds weird, yes, but it's actually delicious.
Haribo Gummy Bears with Juice
A gummy varietal that does not export to this side of the Atlantic: the Haribo bears "mit saft" (with juice) don't taste like anything other than...I don't know, really juicy gummy bears. But there's supposedly real fruit juice in them because Germans are sneaky and unconventional.
I hesitated to put this one on the list because apfelschorle is literally just lightly carbonated apple juice, but for some reason, it's a specific thing in Germany. Not that I'm complaining, it's also really good and a nice alternative to actual soda.
Have you ever been eating cheese puffs and thought, "gee, these are good, but I wish they were peanut-flavored instead"? Well, evidently, a German once had that thought, and henceforth gave the world peanut puffs. It seems a little unusual, considering peanut butter is not a popular commodity in Germany, but these do, in fact, exist. They're airy, but also salty and taste as though they're covered in unsweetened peanut butter. Not everyone's cup of tea, but could be yours.
Another thing that is an actual Thing in Germany is mixing your sodas. (Actually, they're into mixing all kinds of drinks, even beer, and lemonade, which they call a Radler and is much less embarrassing to be seen with than a shandy in the US.) But sure enough, a bottled mixture of cola and Fanta is absolutely something you can buy in Germany. In case, you know, you've ever craved that.