We made a detour trip to Izu Peninsula on our way back to Tokyo for one night stay at a traditional Onsen inn, Kira no Sato. It's a huge treat for us to visit Onsen in a local area of Japan and something we aspire to do whenever we can while visiting Japan. I did spend quite a bit of time investigating which Onsen to stay that was convenient enough between Tokyo - Shizuoka transit. I was happy when I came across this one that had a large enough property for kids to explore when they didn't want to be inside of the room, an option to have our space to avoid waking up other guests during our jetlag days (have been up since 3am for last few days...), traditional Japanese aesthetics with updated amenities (like easy to use clean bathroom). Usually, Onsen stay comes with curated dinner and breakfast with local flavor and that's also a fun way to experience different regions of Japan.
So if you are ever in Tokyo area, this little Onse is worth the 2 hour trip via Shinkansen and local train!