Thanks all so much for the replies!
I'm not particularly interested in a van tour for this part of the trip. I appreciate local insight but want this to be a "just us" day.
To be clear, if we did Eguisheim, I was figuring on a quick Uber back and forth rather than waiting for a bus. And we do want to spend some time in Colmar itself.
The Kutzig hop-on-hop-off is really interesting! But it seems like the route would take us through other towns that we weren't going to do on the way to Ribeauville. It would be nice to see more towns (even just from the bus) but I'm not sure if it will actually save us time and that's my bigger concern. Unless I'm misunderstanding their route map or the timing between towns, which is 100% possible. Maybe I'll reach out to confirm with them.
Denny, it's good to hear from someone that the driving was easy and parking not bad. I have a memory of driving down a street in Auxerre and being VERY worried about our passenger rear view mirror. But stashing the car at the edge of the historic part of town and walking from there sounds good.
Gunderson, THAT is a really interesting idea and might be my favored option right now. I had crossed off Riquewihr because it sounded a little over-touristed... but it also sounds beautiful and a walk between the towns by the vineyards sounds awesome. Do you have a sense of how long that walk would be? I'm seeing an hour and change but need to find something that describes the route better - not sure if that is a round trip time when we would just be looking to do it one-way. With this plan, the transportation would be an early train from Strasbourg to Colmar, a bus from Colmar to Riquewihr, and a bus from Ribeauville to Strasbourg... three legs sounds less scary than four. Or maybe I reverse the order so that the buses (generally less reliable) are at the beginning of the day and the easy train ride home (fingers crossed) is at the end.