Cucumber: I don't recommend cooking cucumber noodles. Enjoy them raw and twirled into your favorite sauce.
Kohlrabi: Saute in a pan for 8-10 minutes.
Summer squash or zucchini: My favorite method of "cooking" these noodles is to twirl them into a warm sauce, as they will get soggy if overcooked. If that doesn't work for your recipe, saute in oil for 1-2 minutes max, keep in mind al dente is best here.