Day trip to Nikko
 

We would like to do a one day trip to Nikko. We may use the Japan railway pass. We wonder if it is better to do it on your own or pay for a tour (expensive$$)?. How easy is to reach there and then move around?.

If you are spending that much time in Tokyo, I am wondering if the pass is worth it.

For your first trip to Nikko, I think a daytrip is too short and hectic. If I remember right, it takes a good 2 1/2 hours to get there, then it's another 30-40min to Kegon Falls if you plan on going there. Then you have to factor in bus times, finding somewhere for lunch, etc.

My trip to Nikko was 3 days, though my goal was more for relaxation and chilling out rather than pure sightseeing.

I'd probably make Nikko an overnight stay rather than trying to rush it in one day for your first visit. For transportation they have buses that go around the area and over to Kegon Falls.

If Nikko is your only destination outside Tokyo, it's not worth the RailPass. It'll be cheaper if you just buy local train tickets.

If you just want to see the massive temple complex, you can do that in a day, no problem. When I went to Nikko on a day trip, I didn't go anywhere except the temples, and I saw everything just fine. It only becomes hectic if you want to see the temple plus the natural features like the falls. (Having already visited Nachi Falls and others, I decided to skip that stuff, though if I had another day there, I would have gone)

You don't need to pay for the tour, I just explored on my own.

You also don't need a rail pass to get there, though if you have one and the days will otherwise be wasted, you might as well use it. But in general, the rail pass is only really useful and cost effective for long trips, especially on the bullet trains. If you aren't going from, say, Tokyo to Osaka or Kyoto or Hokkaido, it's probably a waste to buy a rail pass.

Tobu Line
 

The Tobu line has a pass for Nikko. It leaves from Asakusa station and has, depending on your needs, passes to different spots. I'd check out the site below and see if it's worth it for you. I learned that from Free Worldwide Travel Guides - Wikitravel. If you type Nikko, some interesting info pops up with some worthy options.

TOBU LINE GUIDE | Asakusa Kawagoe Nikko | Japan