There is no reason to go to Nii, unless you want a nice walk in mountains and countryside. There is also a very pretty lake up in the mountains. Not a tourist attraction.
Take the JR to Himeji and change to the Bantan Line. Nii is an express stop. From the train station turn right and cross the tracks when you can. That is probably the main road. Once you cross the tracks you will come to a river; cross that as well. Then walk uphill. There is a main road going that way but next to it is a country road going the same way. After about 30 minutes you will come to a park-like area and a dam. You can walk around the lake above the dam.
photo by 663highland