Sorry these web sites are all Japanese-only:
店舗のご案内 | 店舗数日本一の風呂屋｜極楽湯
Public bath type onsen. Closest from the Mishima station.
http://www.yado-sagashi.com/hp/kinsyoukan/room.html
Basically a hotel (onsen-ryokan) but bathing only is also available.
日帰り温泉の湯郷グループ｜湯郷三島
Public bath type facility.
http://www.manyo.co.jp/numazu/
A bit far from Mishima station but internet etc. is available.