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Haruna-fuji cable car runs frequently. Maybe every 30 minutes. If memory serves me it's around 1000 yen to get up and down. You have the option of going only one way if you want. The whole experience could be done in about 2 hours if you really take the time to take it all in. I've been up several times, and some days it's spectacular (one day I could see the towers in Shinjuku!)... some days its mediocre.

For Myougi-san there are a few stone arches, the lowest one you can get to with just walking shoes, but most of them require a bit of scrambling with the chains mounted into the rock face. It can be a bit hairy for some people, and no problem for others. I usually went in sandals, but always got strange looks from Japanese people.

Megane-bashi is definitely short photo-stoppable. If you want to walk around the area it is really nice, but there are lots of nice things in Gunma. (and many Megane-bashis in Japan)

Jigokudani is best done in winter, although in summer I hear they open the road. It's a bit of a hike up to the park. From Gunma it's definitely a full day trip. From Kanbayashi Onsen it's about a 30-40 minute walk to the park entrance, and from there another 15 minutes to the place where the monkeys usually hang out. Again, I've only been in winter, so it might be less in summer time.

Daruma-ji is good for an hour or less. There isn't much to see, honestly.

Very helpful info. Thanks!

I'm planning a driving trip with friends thru Gunma, Nagano and Gifu so the monkey onsen will be on the way between Kusatsu and Nagano. I'm trying to figure how much I can pack into a day.