Recommend place after Nokogiri Mountain?

I will be making a day trip from Tokyo and have decided to make a visit to Nokogiriyama in the morning this weekend. Apart from this, where I should visit next on that day itself.? Any recommendations?

It would share help if you gave more info, like when you are traveling, and your interests.
Not too far away from Nokogiriyama is Mother Farm.
https://www.japan-guide.com/e/e6432.html

There are a number of flower places at the tip of the peninsula.
https://www.japan-guide.com/e/e6435.html

Narita actually has a few great temples. There is also the Boso no Mura for a look back to the past.
http://www.chiba-muse.or.jp/MURA/e/index.html
You could also try the Sawara Canals.
https://www.japan-guide.com/e/e6406.html

In Sakura City is the Sakura Furusato Hiroba, and in April has 600,000 tulips, in July 20,000 sunflowers, and in the autumn 500,000 cosmos flowers.
http://bit.ly/2GYKPRy
And in Narashino there is the Yatsu Rose Garden plus in Yachiyo there is the Keisei Rose Garden; best in May or Nov.
http://www.keiseirose.co.jp/garden/images/index/whatsnew/2016.10_engli...

No shortage of good places.

Will you have any time left in the day since it is a 105-120 minute train trip each way plus bus to mountain. Or you using car?

Actually yeah you would. Travel and about 2 hours walking the main paths will be about 6 hours.