Sorry, but that's not such great advice. The OP is in Japan for the first time and only there for 7 days and is planning on using the JR pass..... then renting a car is a rather silly option, not even considering that they will be fresh of the plane.

The question should be why a (day)trip to Nikko in the first place. There's plenty to see in Tokyo and Kyoto for those 7 days. And there are other decent day-trips to be made, starting from Kyoto might be the more convenient option.... but it's really for the OP to decide what will interest them.