To rent a minibus in Munich is not at all a problem. There are firms for renting buses and minibuses. You will probably need a minibus. This site has minibus hire munich where you can find out the rental price, short-term and long-term rental...
If you and children will stay in Munich for three weeks, then you can visit many historical places, the sights of the city, for this, of course, it will be profitable to rent a minibus for a long time. I think that in Munich there is not one company...
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