Is one night two days at nikko too much?

Hello,
I'm a huge hiking-walking lover, and I decided to pay Nikko a visit. Mainly for the Senjougahara Marshland. But I also noticed it has some natural attractions such as mountains, lakes and waterfalls. + the road takes some time from Tokyo to reach Okunikko, so I was wondering : would spending a night there be worth it ? Does it have enough things to fill 2 days's worth ? If yes, can you please tell me what and how to get there ?(other than Senjougahara Marshland), I would really appreciate it.
P.S: I'm planning to buy the all area 2 days pass anyway

Yes, I actually recommend an overnight stay if you want to hike Senjogahara.

We once visited Okunikko starting from Tokyo in the morning, getting to Nikko by train and then taking the bus all the way up to Yumoto, where we started the hike down through Senjogahara to LakeChuzenji. That took most of the afternoon (i am not particularly a fast hiker). Stayed at a hotel right by the lake, the next day we walked partly around the lake, then took the bus down to the World Heritage main temple area. If you are well prepared, you can climb Nantai-san.

Thank you for the answer !
I ended up cancelling my stay at Tokyo this night and reserving at Nikko. I'm now on my way there. Will probably do the opposite of what you did. I think I will visit the World Heritage Area today, and after resting this night, will do the hike and Senjougajara from early in the morning.
Tho I have to admit that climbing Nantai-san is indeed tempting. I am actually quite prepared. I have everything needed. Could you tell me how do you get there ? Thanks again !!

If you are traveling from Tokyo to Nikko around lunch time, then yes the sequence you are planning on sounds good.
There is a bus that goes up to Yumoto, so tomorrow morning take that bus up to Yumoto and start the hike down, stopping to view some waterfalls, down to Lake Chuzenji. You can also visit Kegon Waterfalls easily from the lake too.

Thank you very much !!
I actually visited Kirifuri No Taki and Hougen yesterday. And today I'm doing exactly as you're saying. I catched the 1st bus now and I'm going up.
Thank you really !!!!

Thank you very much AK for your suggestion. We are planning to visit Nikko in autumn and we shall follow exactly the itinerary you have suggested.