I am planning a trip to Croatia. I have an overnight flight that lands at MUC in the morning and plan to rent a car to drive to Split. It's about an 8 hour drive. Since I really don't sleep much on the plane, I plan to do about 4 hours driving the first day and get a hotel in Ljubljana.
What sights are there see (excluding Salzburg because I've already been there) between Munich and Ljubljana that are not too far from the main highway? Would Grossglockner pass be too much of a detour?

Firstly, 8 hours is extremely optimistic for the drive from Munich to Split. I have driven a similar routing (Bregenz, Austria to Split) and I would say that Munich to Split is a minimum of 10 hours + breaks.

I don't know about Grossglockner, I'm afraid. Although I live in Austria, I have never actually been there.

As for interesting things on the way, I would recommend either Lake Bled or Lake Bohinj. Both are a short distance north of Ljubljana. Which one is better is a matter of personal preference. Bled is more touristy and developed, whereas Bohinj is more rustic and natural.

Indeed, Google Earth suggests an almost 10 hours long drive from MUC. Also, consider a potential delay at borders, including Schengen ones (Croatia is not part of it), if they decide to go for checks as they recently did between Austria and Germany.

My ending destination is closer to Sibenik, so that cuts about an hour of total time, but even if it is more than 8 hours, I only plan to drive to Ljubljana and not Split in one day.

The Gro�glockner Hochalpenstrasse (the 107) is fascinating. I have driven it several times (both ways) and I highly recommend it. Having said that, it is going to take you far out of your way. I would estimate that from MUC to the start of the 107 you're looking at something like three hours. The full length of the road will take about two hours, but you'll want to stop multiple times for the views, photos, etc. You should also stop at the mountain outlook itself (though this can get very busy in the summer). So I'd figure three hours. It is a stunning drive. From the end of the 107 to Split you're looking at another seven hours or so. So overall you've got at least 12 hours of driving if you want to go the Gro�glockner way.

Also, you should ensure that your rental agency will allow the car into all of the countries you're passing through.

Don't forget your vignettes

Approximate vignette costs

Austria: 10 days, �8; two months, �23.4; one year �77.8

Bulgaria: 7 days, �5; one month, �13; one year �34

Czech Republic: 10 days, CZK 310; one month CZK 440; one year CZK 1500

Hungary: 7 days, HUF 2975; one month, HUF 4,780; one year HUF 42,890

Romania: 7 days, �3; one month, �7; one year �77.8

Slovakia: 10 days, �10; one month, �14; three months, �13, one year, �28.

Slovenia: One week, �15; one month, �30; one year, �95.

Switzerland: One year, CHF 40

You don't need all these of course but I cut and pasted from my road trip and could be of use to others

I am only planning on driving to Ljubljana and not Split the first day. But Grossglockner sounds like it's almost a whole day trip. I checked with my rental agency and I'll have to pay 35 euros to take it out of the country and Croatia is on the list of allowed countries, I just can't take Audi, BMW, Mercedes, VW into it.

Thanks for the Vignettes, I know I will need them for Austria and Slovenia. Croatia does not have them yet, but they do have Electronic transponder system.

The last time I drove in Croatia (just over a year ago), you pulled a ticket at the start of the toll section and paid by cash or card at the end, similar to the system in France.

Is this trip in the summer? If yes, calculate for a LOT of traffic going south to Croatia, especially bad on Saturdays and even on Sundays. The main alpine crossings (Austria's A10, A9 and A13) see hour-long jams every summer, and the Croatian border will also cost a lot of time even though they are just waving through basically everyone.

It can be recommendable to take the Felbertauernpass B108 to cross the Alps, which is both very scenic and usually not jammed (often faster than taking the direct way). If you take that route, a short stop in the town of Lienz can be nice, it's got a small but nice old town. Zell am See, Kitzb�hel or any of the Carinthian lakes can be very beautiful and are mostly along the way. The Gro�glockner Pass is definitely also an option, but will cost you a bit of time since it's not exactly made for fast driving and in the summer months tonnes of motorcyclists and tourists drive that road.

If you're planning a one-way car rental make sure the agency allows it.

Also remember that, even if it is allowed, cross-border one-way drop fees will be astronomical.

I'll be driving from Munich in second half of May, so the tourist season should not start in earnest by that time.
This is not a one way rental as I will be returning the car to Munich.

I'll be renting a car in Vienna and plan to drive around the Austrian Alps. Does Hertz provide the Vignette or do I need to purchase one before I get the car. If so, where do I purchase the Vignette?

Since you're renting in Austria the sticker will probably be there. If not, any petrol station will have it.