[samurai007]

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.