Here's the House Rabbit Society's suggested veggie list, which says to give bunnies at least 3 different types every day: http://www.rabbit.org/care/veggies.html
Note that parsley, mint, cilantro, and basil are all on the list.
Flash's favorite is cilantro, so I usually give him that and parsley every day. I give him kale sometimes, which is another favorite, but that list says to give kale sparingly, so I've been cutting back. But if you're having a hard time giving Opus leafy greens, I'd recommend trying kale. My sister and I have never met a bunny that didn't like kale.
Most of the other veggies I give Flash aren't exactly leafy - celery, green pepper, broccoli, and carrots being the most common, just because I often have those around the house for myself. If you do give him celery, cut it up first. The "strings" in it can get stuck in their stomachs.
I've given him spinach and endive in the past, but spinach is the only veggie he's ever left in the bowl (though he ate about half of it), and I can rarely find good endive at the store. I wish I could, though. He seemed to really like the endive the one week I gave it to him.
Flash will really eat just about anything though, so I'm pretty lucky with him that way.
Also, are you giving Opus hay? All rabbits should get unlimited hay (usually timothy hay), as it helps with their digestion. Here's the full diet FAQ at the HRS web site if you haven't seen it: