Most American tourists will go down the Rhine, and be perfectly happy with that. But that's because they don't know about the options. The weekend you're looking to fill is full of options. For example there will be a major festival in Mainz, the Johannisnacht. https://www.mainzer-johannisnacht.de/
There will be dozens of wine tasting events at local vinyards. https://www.rheinhessen.de/weinfest/4?tx_sfdeskline_pi3%2525255Bseoid%2525255D=Mainzer-Weinsalon_Mainz5&dateFrom=01.01.2024&dateTo=31.12.2024&eCategoryIds%5B0%5D=23&eCategoryIds%5B1%5D=12
There are small towns that are not on the Rick Steves routes that are really nice. This last weekend I took a carload of newcomers to Germany to several and, while it's not in season, we were the only Americans in town. We stopped at one place in Neuleiningen, got a glass of wine each, and the owner brought in some pork he had on the grill just so we had something to go with it. Nobody goes there, but the view from the castle ruins is spectacular.
Freinsheim is a medieval walled town that still retains it's wall. It's small, you can walk from one side to the other in 15 minutes. But you can also walk completely around the wall. We were there for the Saturday morning market, spent some time watching the storks nest building on the church roof, and had a great meal in an uncrowded hole in the wall.
Deidesheim is another excellent place to spend an afternoon. It's neat, clean, historic, and has three Michelin restaurants and numerous wine cellars and bars.
There are a lot of small towns that are easy to reach by train, and even some of the bigger ones like Bad Durkheim, Neustadt, & Bad Kreuznach don't get much love from Americans. That's too bad, because they are a lot more interesting than the places along the Rhine, and they all have castles.