Onsen in Tokyo

Hi, will be heading to Tokyo again next month, this time with a group of friends. Our schedule will be packed tight but we would like to squeeze in some time for a visit to an onsen. Any recommendation for one that is not too far from the city? Do we need to book in advance? I googled and saw the one at Nikko but are there any nearer? We are staying in Shinjuku and will be visiting on a weekday.

There are some onsen inside Tokyo, but their water is not good. Still, some of them are interesting and nicely done, such as Oedo Onsen on Odaiba and LaQua next to Tokyo Dome City.

But for the full experience, I recommend an overnight stay at an onsen ryokan outside of Tokyo. The most famous and probably most easily accessed one is Hakone:
http://www.japan-guide.com/e/e5209.html

But for even better waters and a nicer town, I recommend Kusatsu Onsen, but it is a little bit more time-consuming to access:
http://www.japan-guide.com/e/e7400.html

Many thanks Uji, for the recommendations. We will most probably head to LaQua because of the limited time.