I will add in my two cents for what they are worth.
If you are into cars, there is a small town south of Stuttgart called Böblingen. (It is where we stay when we are in the Stuttgart area.) There is a place called Motorworld. It is on an old air strip and is a place where people store their extremely nice cars and it is set up like a museum. My hubby and dad were like little kids in a candy store going through it. There is also a cool Biergarten there called
We love Tübingen, mainly as my hubby's family is from there. It is a fun little college town. We did do the punting. We left it to our relatives to punt, but it was so peaceful being on the River.
Esslingen was also really cool. We were there during December and they had a Middle Ages Market. So different and cool!
Right outside of Heidelberg is Neckargmünd. I stayed here during a 6 week summer abroad program in college. We did do a cruise from here. I don't know if it is still available or not.
Aalen is not too far from Esslingen. There is a really cool Romen Limes museum there. I did hear it was under construction, but I don't know how long that will be going on. The museum is in German, but they have headsets with an English translation.
Not too far from Aalen is Nördlingen. Which is a walled town that is almost a perfect circle. There was a meteor that crashed in the area and the town was built on the edge of the crater. We climbed the 400 plus steps to the top of the church. Totally worth it. There is a cat who lives at the top! There is a Crater Museum there - again in German, but if you ask the docent they have an English translation. This is one of the places that we were asked where we were from and the people asking were astounded that we were from the US and wanted to know how we had heard about it.
I know Aalen and Nördlingen are on the outer reaches of a day trip, but they are interesting, so I wanted to throw them in too.
Our next trip back to the area (for a family reunion) will be focused on Schwäbisch Hall and Schwäbisch Gmünd and Ludwigsburg. I am excited to explore these towns as the pics I have seen are so beautiful.