Talking with my family today, I find out that durring our trip to Irel
and durring Christmas this year, we will be able to spend 1 day in Germany. (Hey I don’t know how either, I’m along for the ride)
Point remains, One day in Germany, I want it to be expedient as possible. What do I need to see? What can I skip?
Exactly where and how long are you going to be?
If you’re only going to be in Germany one day, it makes a big difference what city you’re going to be in. Will you be spending the night, or is it just a really long layover?
Whatever you do, drink some beer.
Then drink some more beer.
Without knowing where exactly you’re going to be, the generic answer would be drink lots of beer.
Frankfurt Airport, take the escalator a level or two down from the arrivals level in Terminal 1 to the trains (U-bahn signs). Buy a one day pass that covers the trains and busses. Downtown (hauptbahnhof=main train station) is only 3 stops from the airport and takes about 15 minutes. Lots of stuff within walking distance, as you would expect from any world capitol.
Should you stay in the airport, there’s a few cafes on the “fifth level” that have a glass wall overlooking the airfield. At the base of the escalators that go up to the cafes there were 8 free Internet terminals. Lots of shops in the terminal on the departure level and shops, restaurants, and a grocery store on lower levels.
Munich Airport, you’re almost an hour away from Munich proper whether by cab or by train. During morning and evening rushes these times increase dramatically. Should you take the train, make sure you’re on the express into the city. Marienplatz is the place to go: lots of history, lots of shopping, lots of eating, lots of drinking. You’ll have to change trains at the hauptbahnhof to get to Marienplatz but it’s only a few stops from the hauptbahnhof. I’ve not been there but the town of Erding is right outside the airport, home of Erdinger beer.
Hamburg Airport is about 45 minutes from the airport by public transport. Unfortunately the trains don’t yet get to the airport. Take the 110 bus to the Ohlsdorf station where you can take a U-bahn train (U1) or S-bahns (surface trains) S1 or S11 to the hauptbahnhof. Again, lots of stuff to downtown.
If the waterfront is more your thing, both U-bahn and S-bahn run there. Ask at the airport’s information desk for the trains you have to take.
Berlin Airport, good luck. There are 4 airports. Try to get downtown to the TV Antenna. There may be a line but it’s worth it for the view. Sightseeing is pretty good in that area also. If that’s not an option, go to the hauptbahnhof. Within a mile’s walk, there’s a zoo, a bombed-out church, major department stores, and a Hard Rock Cafe! Next to the church is a plaza where there’s always something going on. Grab a spaetzel, a bier, and a seat for some world-class people watching.
Well, those are the only German airports I remember much about. Enjoy your time.
Like the others said: it depends on where you touch down, and the duration and time of day. Germany isn’t that small - 2.3 times the area of Georgia (US state - where your Location field says you are) and 10 times the population.
You refer to “during Christmas”. If it’s exactly on Christmas you should take into account that 24th December is informally half a public holiday (almost everything closed down shortly after noon), and the 25th and 26th are full public holidays. Between Christmas and the New Year (31. December half-holiday, 1. January full holiday) quite a few sights and venues are closed too - you should look on their Web sites beforehand.
Don’t mention the War - I did once but I think I got away with it.
What?
Since you’re seeking opinions more than facts, I’ll move this thread to the IMHO forum.
bibliophage
moderator GQ
I’ll assume you’ll be in Frankfurt as a transit point. Now, if you’re in Frankfurt am Main proper, you’re fine. There is another Frankfurt airport, if you’re flying on RyanAir (which is quite possible with the whole Ireland thing), which is out in the middle of nowhere. Can’t help you much on that one.
Back to the original assumption: do as CP mentions and get a ticket to ride on the rails. Take an S-Bahn train, from below baggage claim, in the direction of Frankfurt Stadtmitte. You want to get off at the Hauptwache stop. Check, because you might have to change trains at the Hauptbahnhof (central station). The Hauptwache stop is the central shopping area. There should be a Christmas Market going on just above, starting next to the Kaufhalle. This area is called the Zeil and if you walk along in one direction you will have all of the stores and shops, along with places to get a bite or a drink or both. Gluhwein is a decent thing to drink because you’ll be outside and it can be cold, along with a nice sausage. Der Zeil runs off for about 1 mile or so from the Hauptwache. The Hauptwache building itself is right there, as well. This is now a sort of cafe with some food. Back in the other direction from the market is the Fressegasse, roughly translates as “pig out alley”. With several restaurants. To the south of this area is the Goethe house, where Goethe was born or lived, I forget which.
If you follow the market away from the Haupwache, just to the right of the Zeil (sorry, I don’t have my maps here or I could give you the street name) and slightly downhill, you will come to the old city center. The St. Paul’s Cathedral is here and one of the only buildings to survive the bombings during the war. This is built on the site of the old Roman ruins and I think you can explore the basement a bit. Off to the right from the Cathedral is the Romerplatz, which has been restored to what it was like before the bombings. Near here is also a Jewish museum.
If you go a little farther along, just across the river, there are a number of museums and galleries. Just past this, but a little harder to find on foot, is the old Sachsenhausen area (not to be confused with Sachsenhause concentration camp). This area has lots of bars and restaurants and is fairly popular in the evenings.
So, between all of the above you should find something to keep you busy for a single day.
Except there is no single hauptbahnhof in Berlin. That’s the Zoologischer Garten Bahnhof.
A spaetzel?That’s like going to the movies and only eating one Milk Dud. What are you, Eloise? Have a plateful!
And more beer, while you’re at it. 
Anywhere near Cologne, see the cathedral. I was in Germany for a week (Dusseldorf), some friends said see the Cologne Cathedral. I was like why? It was worth it.
take care
robz
Opened the thread solely to make this comment and ya beat me to it!
I second Cologne Cathedral if at all possible, which seems unlikely.
You must eat too: Pretzels
spaetzle: something you spit out 
Munich resident speaking: As a tourist, you’re almost always better off taking the train from the airport. If you’re travelling in a group there’s an all-day-all-lines ticket (AFAIK either for two people or for five) that’s a better deal than single tickets. There is no express train, though, there’s just two lines that go in opposite directions but they both go to the main stations Hauptbahnhof, Stachus and Marienplatz, so you don’t have to change trains.
If you’re going Christmas time the Christmas market is at Marienplatz - lots of tourists but quite nice. Usually they also put up an ice rink (either behind the town hall at Marienplatz or at Stachus). For a nice cup of coffee go up to Café Glockenspiel at Marienplatz (entrance opposite of the department store Kaufhof) overlooking the Marienplatz and - you may have guessed it - the Glockenspiel (which is rather not worth waiting for). Right around the corner there’s also the Viktualien Markt, a market where you can acquire all sorts of yummy goodies and also the tower of Saint Peter that you can climb (lots of stairs!) for a great view of the city.