thanks all for the suggestions.

I used Messestadt Ost park & ride; it is indoors and cheap, 1 Euro for the first day and 3 for the following, while parking in the city center is around 20 Euro. Just a 100 meter walk to subway stationU2. Drive to Main station is a bit long (20+ minutes), but it is direct.
Park & ride is actually usually just for up to 24 hours, exceptions are Messestadt Ost (large exhibition area) and the Allianz Arena (stadium of Bayern Munich)

The "Umweltzone" the area you can only enter with sticker is actually pretty large and my GPS guided me through this area; but the signs are very well visible so I did not enter and drove a different way. Saw several other foreign cars without stickers in the zone. Not sure whether control is very tight but I would not try it!