I think broccoli and cauliflower might toughen the consistency a bit ... I haven't seen them used in any of my green smoothie recipe books. I think I have seen cucumber used (seeds removed) but sparingly. Technically cucumber is a vine fruit and not a green, but still great for you.
What I have seen used (and have used myself) are green herbs like dill, parsley, and cilantro. I make a killer savory green smoothie using fresh and sun dried tomatoes, 1/4 avocado, lime, water, pinch of pink salt, kale, and tons of fresh dill (I use a high speed blender to get it totally smooth). It's a Boutenko recipe.
Flat leaf parsley is probably the most neutral of the leafy green herbs. I have seen recipes incorporating cilantro with lime and melon (any type) as a recommended combo.