Thanks for the reply Michigan. No thanks to everyone else
(Seriously, I haven't had much luck with japanese-related forums...maybe they're just full of people who have never been?)
For the sake of others who read this post later, I've decided just to do Nikko anyway. It seems to have everything I had on my checklist - greenery, waterfalls, cool climate, temples, onsen, ryokans. Hopefully there won't be a lot of tourists around as I will be going just after Golden Week. So hopefully most Japanese tourists will be back at work having already taken their holiday.
Prizm