Spinach and kale are both great but you usually need to start slow and build a taste for them. They are not the sweet taste a lot of people are used to.
I've read lots of advice on how to juice them without clogging your juicer or not getting any juice. What works best for me is to wrap some around another food, like a carrot, and juice it together.
If you like lemonade, one of my favorites is 2 apples, a cucumber, 4 leaves of kale, a lemon and a handful or two of spinach. Sometimes I do only kale or only spinach and just add more.