First Europe Trip: Is my itinerary reasonable?

You might have to drop Trieste.
Head thru the Brenner Pass to Italy from Innsbruck. This would work, and driving be best...
Vienna (3)
St Polten (3)
Hallstatt (1)
Salzburg (2)
Schwangau for Neuschwanstein (2)
Innsbruck (1)
Venice (3)
Ljubljana (2)
Budapest (3)
Vienna (1)

Looks ok at first glance. Work up your mileage from town to town with via michelin that should help alot. It will give u approx travel time and mileage as well as any tolls to plan for.

Start in Venice and end in Budapest.......
Of the places I know:
Venice two Full Days
Ljubljana one Full Day
Vienna two Full Days
Salzburg one Full Day
Budapest 3 Full Days

A full day is when you wake up and go to bed in the same town.

Remember.. when someone says " a full day " they mean two nights.. so "two full days" means three nights etc..
I would take trains over buses any day.. but have not done all your routing/

Hi Arizona, good idea to start planning now (I'm a planner too, so to me it's part of the fun!). Fly "open jaw" - fly into your first city and out of your last - you'll waste time going back to Vienna. It does look like a lot of territory to cover in 3 weeks. Maybe make a list of your family's priorities and focus on them first. Have a great trip!

I don't think open jaw is an option here as they are flying in to Vienna to meet up with friends who live in Austria.
It also makes sense to return to Austria with a rental car to avoid excessive drop off fees.

Make sure you get all necessary docs for driving in and out of Austria. Might be worth renting a car in Austria if you want to tour there, turn it in, and rent another after you visit Venezia rather than pay rental and parking fees where you can't drive.

If you were using public transport, trains don't cross the border at Trieste (they go the long way), so you might choose a DRD bus that runs direct from Venice to Ljubljana. Although all of these destinations are served by train, I'm not opposed to your driving plan. For a 3-week rental from Vienna airport, Auto Europe is suggesting an Economy manual transmission car for only $600 through a local company. Automatics cost more. For a period of 3-4 weeks, a car "lease" program may be suggested, but those are primarily through French companies (Renault and Puegeot) and don't offer pick or drop in Austria.

Thank you so much for all of your responses! It's really putting my trip into perspective and made me realize that this will take a lot of studying and learning of the different laws and policies for vehicles, what and what not to do etc. if anybody else has any more driving tips or must see locations that are on this route I am more then happy to hear them.

Make sure you get all necessary docs for driving in and out of Austria.

Also note that Austria is one of the countries that explicitly requires an International Driver's Permit (a multi-lingual translation of your US driver's license). Get one at AAA.