Spending time in the Parisian Vietnamese quarter is a fun thing as well. While Paris is known for being an expensive city, eating at a restaurant or deli in this part of the city was extremely economical, as well as REALLY REALLY tasty
There's quite a few things in Germany as well ... a lot of it will depend on how you want to travel. If you get a train pass, it will not take you very much time to hop from area to area - that will open up your options quite a bit.