If I were a Nikon sales rep I would say you MUST have... nm we don't need to flog that dead horse anymore..
Definitely lens hoods.. as long as possible without getting in the image!
If you shoot outdoor and want to try long exposure shots of clouds/water etc.. you should look into a Circular Polarizer, an neutral density filter or two as well as a tripod.
Another cheap handy addition can be the cordless remote for wildlife or long exposure shooting, self portraits, kids portraits, your own family portraits etc..
A flash can be handy no matter what kind.. almost anything can outperform the pop-up flash! Depending on what your shooting even a $50 macro LED light ring can do a lot.
I tried a pop-up flash diffuser (Gary Phong Puffer) and it did it's job and softened the light.. but nothing compared to a real flash when I bucked up for the Metz 58-AF2.
Ohh yeah.. and of course, some inspiration to go shooting is the most important! But it sounds like you already have that!