We found an ACSI site in Neckarmund about 6 miles outside of Heidelberg where we spent the next day exploring Heidelberg. A bus ran into town every 20 minutes from outside the site which we took into Heidelberg. There was also a boat running from outside the site which we considered but as it was 3 times more expensive than the bus we decided against it.
The old town of Heidelberg is a nice place to stroll around and we had a really lovely day. Its really typical of an old German town with lovely old buildings, cobblestoned streets, large squares with ancient churches, castle ruins on the hill overlooking the city and history around every corner
I worked on a market research car clinic years ago in Heidelberg and always remembered how magical the castle looked perched up on the hill and wanted to come back to explore more.