Traditional- Ein Prosit, In Munchen Steht Ein Hoffbrau Haus, Hi Le-Hi Lo, Du, du liegst mir im Herzen.
Other songs that work well- Pennsylvania Polka, Edelweiss and Do, Re, Mi from Sound of Music. The Happy Wanderer, O du lieber Augustin.
Believe it or not, apparently in recent years Country Roads and Sweet Caroline are very popular in Germany at the fests.
Hope this gives you a start. I'll try to come up with a few others