I have been fishing the Nis all spring and there are some good fish in the FFO section. Unfortunately the water is extremely low right now so if I were you I would hit it at dawn or dusk. There is a good hatch of tiny black caddis this time of year that I would try to imitate if the fish are rising. My advice to you would be to go as small as you can stand with nothing heavier than a 6x tippet. Maybe try a dropper rig.
You might be better off checking out the Squan if you're planning a day trip. There is much better shade/streamside cover and I've caught a couple of lunker holdover browns there before. Just don't forget the bug spray.