CALCIUM - You can kill with kindness!!
As an occasional treat I will buy my buns curly kale (usually on offer as its expensive). Its also nice for humans - steamed or boiled like cabbage. But again it is very high in calcium so must be a rare treat. Rabbits milk drops also have high levels of calcium.
Calcium is a serious problem as it causes calcium deposits (crystals - think lime scale) to build in their bladdders, the crystals work their way into the urethra (wee wee tunnel) causing discomfort and bleeding.
I learned this the hard way within eighteen months of having my two girls Smudge & Tux. I gave them lots of kale and milk drops and i lived in a hard water area. One day I saw blood in Smudges wee, she had to have a general aneasthetic and her bladder flushed. The discomfort she must have been in was awful and to add insult to injury...it cost £250! That was approx 8 years ago, so with inflation I hate to think what it would cost now. I got insurance quick smart, cause I knew Tux was next. My buns went from 5milk drops a day, (spoiled) to 5 a week. And kale rarely. They never had any more issues from then til their final days. Bless their furry ears.
So everything in moderation. These days I only have Harley and he has no shop bought treats but has hay and pellet in his hutch at all times. Grass and dandelions and fast as they grow, carrots, red apples and celery regularly and sometimes dried wholemeal/brown bread and cabbage/cauliflower hearts/leaves. The fruit and veg I restrict to no more than one of each a day. This will always depends on the quantity they are used to. I have always been advised never to give lettuce, altho someone gave my buns a little romaine once and no harm done.
And always give when fresh. My G'dad gave my childhood rabbit veg that was passed its best while we were on holiday, fortunately we came home and got to the vets in time to save her. I dont know who felt worse G'dad or the bun.
These are just my experiences.