Honeymoon/visit kids in Stuttgart, Germany

"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