Nikko in one day

Hello, I've been back in 2007 to Nikko but I could only see Toshogu, the Red Bridge and Rinnoji temple. This time I have only 1 day from Tokyo - with a friend who never visited Nikko, I am thinking to visit just the Toshogu and the Kegon falls and Chuzenji Lake and come back with public transport with JR Pass, do you think is too much? or should I just focus only in Toshogu and Rinnoji and Taiyoin?

I went to 3 places for a day trip on a bus tour from Kanagawa prefecture.
(Since it was a free tour, I stopped by a jewelry shop in Saitama and a hotel in Kinugawa-onsen first.)
It depends on the departure time, but you can take a day trip from Tokyo. (I left around seven in the morning.)
Please check the bus timetable here.
https://www.tobu-bus.com/pc/area/pdf/nikko_timetablenatsu.pdf?2024
The Kegon Waterfall is a 5 minute walk from the Cyuzenji Onsen stop.
Here is the opening hours of the elevator in Kegon Waterfall.
https://www.nikko-kankou.org/spot/5
If you just look at it from above, you don't have to take the elevator.
I arrived about five minutes before the closing time of the elevator, so I didn't take it (I have probably taken it before that).
Please note that Toshogu Shrine also has its opening hours.
https://www.toshogu.jp/etc/haikan.html

Depends on what time you start, but visiting the lake, Kegon Waterfalls and some of the key temples/shrines in one day from Tokyo is well possible. We�fve done it before, and just the other day a friend of mine did a similar trip too.

I did essentially what you are proposing. It is doable if you start early and rent a car. In my case I got there early but went by bus, and it was not so convenient or time efficient. There were not many buses and they took a long time to get to Okunikko and back. The next question is what exactly you want to see or do around Lake Chuzenji. There are a number of interesting places around the lake, but if you want to quickly and conveniently hop on and off a bus to go see them, it's just not going to happen.
Since time is short, I suggest you stick to the central Nikko area. Toshogu is the crown jewel, but Taiyuin is also very good, plus there is Rinnoji as you said, and some other new places for you too like the Kanmangafuchi Abyss, Tamozawa Villa, and Futarasan Shrine.

Easy to visit temple, lake and falls in a single day from Tokyo. My preference is cover the temple first and the lake/falls in the afternoon. There are bus tours that cover the area as well if you want to avoid the trains.
Make sure your rail-pass is a sensible idea as they are expensive these days and the Tobu alternative is cheaper than JR to Nikko.