Many vegetables can be used for tempura. I have tried potato, okra, green bean, bell pepper, yam and carrot. For potato, yam and carrot, peel and slice into thin pieces, 1/8 of an inch thick. Dip into batter and deep fry.
For green bean and okra, dip in batter whole and deep fry.
Onion and dry shrimp is what you don't find in Japanese restaurant. It is really delicious. Slice onion into thin pieces and mix with batter and dried shrimp. Batter should just code the surface of onion but not soupy.