Originally Posted by evergrn

Kusatsu is great, and it's easiest to get there from Tokyo by highway bus. Great onsen, with a vibrant onsen town feel. But your experience will also depend on where you stay and how much you spend. Up in that area, Manza is also well known for great onsen and moreover the high altitude up there makes for an ideal spot in the summertime (Manza Prince offers bus pickup from Karuizawa station). Shima Onsen's probably a bit off the beaten path for foreigners, but is highly-regarded. Ikaho Onsen has that quintessential onsen town feel, although not easy to get to by train. I really love Hakone, and it's the best onsen area that's easily accessible from Tokyo by train. But there're many hotels/ryokans in Hakone to choose from, and the onsen quality varies greatly depending on where you stay at. There have been many discussions on hotels in Hakone on FT. Atami and Ito have lots of onsen places, but again the quality varies from one hotel to another. There're lots of great onsen places elsewhere throughout Izu peninsula, but some of the best ones are in parts that are not easy to get to (eg, Sagasawa Onsen).

Anyways, making onsen recs is never easy because there're so many choices with so many variables including your budget and room availability (which will be a major issue if you're looking at a mere 2-3 weeks down the road). I would encourage you to do your onsen stay between Sun-Thu for better availability and rates.