No specific suggestions for the D750 but on occasions you may want to …get rid of the big heavy lens and go out with a just a prime 50mm lens with the camera set for manual use. Slow down, have some fun and over time your camera knowledge and technique will improve. I enjoy experimenting with manually settings aperture, shutter speed, and ISO when taking a shot. Not for everyone but I found out it slowed me down and got me to think more about the picture I was taking then firing away,
Congratulations on your new camera, it has certainly has received many positive reviews since it came on the market place.
I would suggest picking up something like the Zeiss Planar T* 50mm f/1.4 ZF.2.
FWIW, there is a thread on Flickr with the D750 and manual focus lenses. Manul Focus on D750 thread