For Russo Loco and his daughter and grand son in Germany - the easiest steam narrow gauge from Munich would be the Zillertalbahn in Austria. It operates one steam roundtrip Tuesday-Sunday from Jenbach to Mayrhofen, Jenbach 10:42 - Mayrhofen 12:14 returning 14:33 - 16:00 Tuesday-Sunday until October 3.The train has enclosed passenger cars as well as an open gon. Jenbach is on the mainline from Munich to Innsbruck, a fast trip from Munich. The Zillertalbahn website has an English translator option. Unfortunately the Achensee steam railroad from Jenbach is not currently operating.
There are other steam operations within reach of Munich - one is the Bavarian Railroad Museum (Bayrisches Eisenbahn Museum) at Noerdlingen (north of Augsburg) that runs steam trains on two routes out of Noerdlingen, even transporting tonnage freights of sugar beets with steam at this time of year.
Another interesting operation (that I haven't personally seen) is the Sauschwaenzlebahn (Pig Tail Railroad) also called Wutach Valley Railway at the southern edge of the Black Forest
that is operating steam on weekends (6 tunnels, 4 viaducts, including the only spiral tunnel in Germany)
Jenbach is at the very bottom of the page, the Bavarian RR Museum is at number 136