Travel from Germany to Copenhagen

I totally agree. Lübeck is your best choice: Beautiful town, not too big and direct train connection to Copenhagen. The other option is Hamburg, which also has direct train connection to Copenhagen (actually the same train, stopping en route in Lübeck). These are the only two possible options which meet the following criteria: 1: city large enough to have several car rental agencies and 2: city with direct day train connection to Copenhagen. Remember that reservation is required on the EC/ICE train to Copenhagen, but this can usually be made on the day of travel.

Hammburg is big so dropping a car should be no problem. Further North, Kiel might be another good alternative. It's a beautiful old city on the sea. Schlesswig is a regional capitol, also beautiful on a fjord, but it's pretty small. Schlesswig is conveniently located roughly 30km from the Danish border; if you can drop a car there that would probably work best.

The Northern-most German city is Flensburg, pretty much right on the border to Denmark. There are two main rail routes between Germany and Copenhagen. German ICE trains run Hamburg, Lübeck (very beautiful town, would make for a good drop-off point) and then Puttgarden - Rodby... Copenhagen. The entire high-speed train in Puttgarden rolls onto a ferry boat for a one-hour "cruise" to Rodby. Even though the ferry ride consumes one full hour this is the fastest connection between Hamburg and Copenhagen (5 hours from downtown to downtown). Danish trains also use the Öresund bridge/tunnel instead of a ferry boat. Personally I'd recommend the ICE train connection from Lübeck to Copenhagen (4:25hrs)...

By the way: Reservation is no longer required on the direct EC/ICE trains from Hamburg/Lübeck to Copenhagen, only recommended.