Senjogahara is a marshland in the Nikko National Park, 3-4 hours from downtown Tokyo. It is an unbelievably beautiful place, especially in autumn.
As with any place around Tokyo, you should expect plenty of people on the weekends and during the peak seasons - spring and autumn here. Both the train and the bus were packed, and during my six hour walk, I was alone for less than twenty minutes total. As a direct consequence of the number of people, there are only a couple of trails through the marshland, which limits the experience. However, after seeing the condition of some less protected places here in Japan, I can only applaud the decision.
The crowds though should not prevent you from enjoying the alien landscape.