"We are just brainstorming now." Oh. OK.
3 nights on the Rhine: It seems that most Rick readers end up in Bacharach. I like Bacharach but less as a base town and more as a place to visit for a short while. You'll find a wider choice of eateries and accommodations in Boppard, a larger town on a huge curve in the Rhine with a nice number of half-timbered buildings and with a bit more to do. There's a pleasant town square filled with outdoor cafes, a small set of Roman-era ruins, a lengthy river promenade, and a nice chairlift ride to a nearby lookout as well, at the top of which, after a short walk, you'll find the Gedeonseck terrace-cafe with a commanding view of the river and town:
(Gedeonseck, slightly goofy photo)
Boppard (on right)
Boppard's tea house
Most of Boppard's accommodations huddle by the river promenade a good distance from the railway and are less impacted by noise from passing trains than places in Bacharach, where the tracks cut right alongside the narrow town for its entire length. Also, grocery stores are more numerous if you're packing lunches for outings or renting an apartment (as many area visitors do.)
Boppard is a good place to end a river cruise as well - so it's convenient to stay there. You would take a direct train south to Bingen, then board a northbound cruise boat and get off in Boppard 2.3 hours later having cruised the most scenic segment.
Boppard is also a slightly better location for heading over to Cochem on the Mosel River, a very good outing if you have the time - terrific old town with walls and towers, castle with falconry show:
Bacharach, Oberwesel (old town walls and towers here too), and St. Goar (home of Rheinfels Castle) are other towns of interest on the same bank of the River as Boppard and can be reached easily as well.