We'll be travelling this September from Munich>Vienna>Salzburg>Innsbruck>Fussen>Munich. I'd like to rent a car in Salzburg to see the smaller towns between there and Innsbruck and Fussen. Does it make more -sense to return to Innsbruck and leave the car there or just continue to Munich Airport? Also, does anyone have a recommendation for a vehicle large enough for 4 adults and 2 kids who will need car seats? Many thanks.
Driving in and around Innsbruck and Fuessen is relatively uncomplicated by European standards, so my vote would be to drop it off at the airport... with the caveat, I assume, that you're only leaving from Munich, not spending additional time within the city.
Before you decide to drop off in Munich be sure to find out if there is a fee for airport drop off and/or fee for international return. It sounds like you will need three rows of seats so some kind of van will be necessary and your choices may be limited.
If you are planning to visit Salzburg for only a few days -why not just rent the car or van at the Munich airport? I don't think you can beat the convenience of picking up and dropping off your car at the place of arrival, especially if you are traveling with a group of adults and children. Also, I'm sure you'll pay a premium for picking up your car and dropping it off at different locations, at least equal to a few days of car rental.
Salzburg is about a 2 hours drive from Munich, its an easy drive from Salzburg to Innsbruck, and an easy drive from there to the Munich airport.
Check with rental car company usually there is about 100-200 drop off fee for another country. It should be cheaper to drop car off in Innsbruck but also need to check how much train tickets cost from Innsbruck to Munich. It might be cheaper to pay extra fee if it is cheaper than train tickets and with two small children car vs train and how much luggage you are traveling with. With me it it is which is easiest. If you are staying in Munich drop car at airport and take sbahn into city. If you want to avoid airport with kids you could stop a ismaning or another stop along sbahn and take kids into city.