I think Kegon Falls should be doable :) We've been on a day trip to Nikko from Tokyo, starting from Shinjuku/Ikebukuro on JR+Tobu train (we live in Tokyo so no JR Pass available to us), took the bus up to one end of the Chuzenji Lake (the end closest to Nikko town) and done a short walk by the Lake Chunzenji, and had a good look at Kegon Falls, then taking the bus down to visit Toshogu Shrine and a few other shrines around it, as well as the cat and the three monkeys and the bridge, at a no-rush, good comfortable pace.
I believe we got to Tobu Nikko station around 9 in the morning, and left for Tokyo on a train departing Nikko around 4:30 in the afternoon.