If you dont want to get another contract, get a simple mobile off EBay that can take a GSM Sim card. When you get to Germany, do as the other poster suggested and buy a pay-as-you-go package and put the SIM into your phone. Do not use an AT&T or T-Mobile phone number overseas, the roaming charges are ridiculously high and it will cost you a fortune. I have had T-Mobile for 2 years and just changed over to AT&T here in the US because my monthly charges were sometimes over $400 due to roaming, and that was in the US, not even overseas. Virginmobile seems to have a lot of pay-as-you-go packages and here in the US they will even give you a basic phone in the package too. I have used Vodafone and Telefonika in Europe and they worked well.
The weather in Summer will be great but for a real treat with a difference, go visit them early in December and visit the Christmas markets in the different towns. I did it a couple of years ago and loved it, really good food, gluhwein and unique gifts. A wonderful experience and different to anything I had experienced before. Enjoy your visit.