Honeymoon/visit kids in Stuttgart, Germany

No need to travel to Octoberfest when you will be in Stuttgart - just choose "Wasen" or Cannstätter Volksfest.

With 15 days in total I would reduce it to Paris (4-5 full days), Stuttgart and Black Forrest. You could add an Alsace tour on the way but be realistic not to press in more. Lake Constance area is near to Stuttgart.

Some more inspiration you can find on tourism website of state Baden-Württemberg.

"The only thing we have planned at this point is going to Paris for a few days..."

A trip to Paris should probably eat up 4-5 days. It's not easy to visit quickly.

"...and to Munich for the Octoberfest."

Accommodations in Munich are quite pricey during O'fest. Also, round-trip Stuttgart to Munich tickets are unnecessary if you take advantage of the Cannstatter Volksfest, right in Stuttgart's back yard. It's Germany's "other" Oktoberfest.

https://www.cannstatter-volksfest.de/en/volksfest/

"Also, we want to go to the Black Forest. Any suggestions?"

Calw can be visited on a day trip from Stuttgart.

Actually, there's a large number of very interesting day trips you can take from Stuttgart.

10 - 15 min. from Stuttgart: Ludwigsburg
22 min. from Stuttgart by direct train: Besigheim
11 min. from Stuttgart by direct train: Esslingen
1 hr. by train from Stuttgart: Tübingen
Towns on the German Half-timbered House Road

"We would like to travel to a couple of neighboring countries like Austria and Switzerland. however, we will be tight on money and all the train rides we have looked at are over $500 a piece (if I read it right)."

Keeping your trips short always helps reduce costs. Not sure how much time you have for additional far-flung journeys beyond Paris. What is it you're reading? Advance-sale tickets from DB are typically a good deal.

https://reiseauskunft.bahn.de//bin/query.exe/en

Ah, I see MarkK had a faster trigger finger with the Volksfest suggestion.

I should have mentioned previousl that if you travel for more than just a day trip to the Black Forest, certain Black Forest towns will make sure your transportation around the Black Forest is free:

https://www.blackforest-tourism.com/info/KONUS

Thank you! These are all very helpful tips and suggestions. I've been lookin gon trip advisor and Rome2Rio.
Michelle Bishop Gardner