What are the absolute best veggies for a beardie?

Ive had my bearded dragon for about 6 months... he is fine and perky.. he looks completely healthy. I feed him crickets and kale everyday.. but ive just realized i never feed him anyother veggies other than kale... I know im suppose to give him collard greens and shredded carrots... but what are the "absolute best" thing that I can feed him? and what is "collard greens"?? my local grocery store doesnt even know. please help.. i want my dragon to be in the best health that he can be. :) Thanks