some onsen charges JPY400 ~ 500 per person. but, since you are going with family, go to onsen where it is a family mixed onsen and this onsen requires you to have swimwear. else, take time out , go alone je lah. got few numbers around tokyo.

Onsen around Tokyo (within 30minutes train)

Dear Guardian n bonzaimy,

for onsen allowing swimwear - refer here

Yunessun - Hakone
go under Mori No Yu

for me , i prefer the open air onsen, traditional one. hehehehe, but not those with a lot of tourist cos it kills the ambience with their laughing and chattering. i just want to relax.

hello bro....apasal u tak cakap u tak pernah pi...i can always go with you ma...drinks on me....girls on you...okie? :P 

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Added on March 23, 2009, 2:42 pm
Yes, out phone can be used in japan...coz i have been using mine there for a week....come back...limit almost break-record liow...hehehe