I do agree that a day trip from Tokyo to Nikko is bit difficult task, because of roads heading to Nikko and rail service it takes longer to get to Nikko from Tokyo than observing distance from a map. If you are planning to do just Toshogu temple only then I think day trip to Nikko from Tokyo is possible. However, if you plan to go up Iroha-zaka and onto Lake Chuzenji then further into Senjougawara and Yumoto area then a day trip from Tokyo become very difficult.

jpatokal is right, Nikko is not particularly known as onsen destination but there are onsen in Nikko area. One is at Lake Chuzenji (Chuzenji-ko) where you find Chuzenji-Onsen (Chuzenji Hot Spring). Going further to Yumoto area where you find Yumoto-Onsen (Yumoto Hot Spring). Looks as Yumoto area has most number of onsen ryokans in Nikko area.

Yumoto Hot Spa