If you have to fly the segment from CTS to HND, then an onsen hotel in Izu might be convenient. If you care more about ease of access from Haneda, then Kamakura might be a good option.

If you don't have to fly the segment from CTS to HND, I'd suggest looking at flights for some of the locations that Always Aisle mentioned - e.g. Fly to Sendai or Komatsu (for Kanazawa). CTS also has flights to Hiroshima, Fukuoka etc. You could also just spend a couple of days in Sapporo or Otaru.