Accommodation!
I've booked myself 3 nights in Nakatsugawa (not cheap by my standards on this trip so far, and I hope the 'smoking permitted' in the guesthouse isn't taken advantage of!). It seems like I should be able to get a train up the valley and then walk some of the Nakasendo trail back over Magome pass, which is supposed to be quite nice. I think I'll have a go at that tomorrow.
Then the weather forecast for Thursday looks pretty stormy, so it may be a good day to rest/plan.