Easy:
* Nine nice new nail nippers.
* Never nod your nose, Nelly!
* No need to name names, Nancy.
* Nine nimble ninjas nibble noodles.
Medium:
* Nancy's new necklace nearly needs a new knot.
* Nathan's nimble fingers never fail to tie knots.
* Nice, neat, narrow notes need no new notes.
* Neglected needles nicked the new notebook.
Hard:
* Ned needed ninety-nine new noses for his ninety-nine new nephews.
* Nine nimble nymphs nibbled ninety-nine nectarines near the north nest.
* The noisy, nervous, naked nymph named Nancy never nodded in the night.
* Near the north, nineteen nervous, noble knights nibbled ninety nuggets.
Have fun trying these out! Let me know if you'd like more tongue twisters.