efficient itinarary

The most efficient will probably be to take the night train from München to Croatia (Zagreb or Rijeka), costs a bit more than daytime transportation but saves you one hotel night.

Have you seen Munich before? Otherwise, why only 1 night there?

Lucerne is great, we loved Interlakken as well. However, Switzerland is very expensive, be prepared to pay and pay.
Then you are headed back to Salzburg. Why backtrack? Salzburg is way closer to Munich.

I see you are headed down to Croatia and Slovenia from Vienna. If driving, you should note that Croatia is very mountainous and roads are not like in Germany or Austria. The best places to visit in Croatia are Dubrovnik and Split on the coast, but driving there is a long drive from Austria.

You might consider dropping your trip to Switzerland considering the rest of your itinerary. Also, spend more time in Munich and Vienna. You are spending one night in Innsbruck, I guess you didn't want to visit Garmisch/Fussen, Germany?

Hi! We are a group of 2 adults, and two teenagers (16 and 14). The
travel length is 20 days (or less if possible). Here is the list of
cities/countries I'd like to visit and need help to create an
itinerary that's most convenient and cost effective.

• Germany: Munich -> 1 night

• Switzerland: Lucerne -> 3 or 4 nights Innsbruck -> 1 night

• Austria: Salzburg -> 3 or 4 nights Vienna -> 2 nights

• Croatia: Zagreb and Zadar , see two national parks and closed by areas, maybe 4 nights

• Slovenia: Ljubljana -> 1 night

In 20 days or fewer "if possible" (not sure if the 20 is inclusive of the days you travel transatlantically), you and your teens will check in and check out of hotels a whopping 7 times. If you seek efficiency as your thread title suggests, I think this is inefficient with all of the hotel transfers, travel, packing and unpacking. This appears not only inefficient, but also exhausting and so speedy, you all could be missing all that you have traveled so far to see & experience.

try plotting this on Rome2rio website for either back tracking, or transport options. You can't necessarily trust the price or schedule, as they don't run on seasonal and dynamic pricing/bookings, but it is a base start...and a reality bite. From there, check the specific transport website.
I would certainly not loop back just to return a car. I would anticipate my teens to giving me grief before then. Rental car for Croatia, sure, but I might be inclined to drop Slovenia and Croatia until you can offer more days. Maybe do a Venice, Trieste, Slovenia, Croatia, Montenegro trip focusing on parks at a later date.

Hi!
Thank you everyone for your feedback. I've realized that I have too much ground to cover with not much time. I'll look at the whole trip and cut back on the cities/countries to visit. Your inputs are greatly appreciated!

There is the night train option from Munich to Zagreb if you're interested in that. Munich is night train hub. I would have no problems using that option.

Easiest and most efficient way to save time is to use a multi-city air ticket to fly trans-Atlantic: For instance, into Munich, home from an international airport in Switzerland. It will save time and maybe money when you consider what it takes to get back to Munich. But you must use a multi-destination search function; these itineraries are not a set of one-way tickets.