Austria is pretty close. Salzburg is +- 2 hours by train and a single ticket costs 22 EUR. Vienna is also within reach. It's 6 hours by bus. With Orangeways you can find single tickets for as low as 16 EUR.
You can also consider a trip to the Czech Republic. Prague is a bit less than 6 hours by direct train from Munich. A return ticket costs 65 EUR. You can also go there by bus. There are several providers. It may be a bit cheaper, depending on when you book and travel. Travel time by bus will be around 5 hours.
Switzerland can be an option too. It is more or less 4 hours you can go by bus from Munich to Zurich. Prices with MeinFernbus.de start at 15 EUR, depending on when you book and when you travel.