No beet or turnip greens, high in oxalates. I avoid oxalates where I can. Mine, given the choice, don't even touch it.
If cucumber leaves taste anything like pumpkin leaves they are safe from rabbits - and when my rabbits leave something alone I respect that. They are not fans of pumpkin, it's an aquired taste, in the garden they just take a test nibble (and spoil the fruit
), but don't eat it, in winter they are not overly enthusiastic about it but in the end of the day it's gone.
Comfrey - read up on that. My approach though is to offer them a huge variety, and they know better than me what they can stomach or not, I let them get this level of experience.
Nettles are great, I cut them, spread them on a sheet, roll it up and knead it to destroy as much stingy hairs as possible, let it wilt for half a day or so in the sun and feed it, or just make hay with it. Last year I collected the seeds when they were big and green, dried it, great treat through winter.
Be carefull about mint and nursing does, it is said to be quite efficient at drying up milk
Half of the stuff I feed I don't know what it is, but when used to a wide variety they themself know best what they can stomach, or how much of it. Ivy and other plants found on many "toxic lists", got eridicated in my yard. Imo they use somewhat "toxic" plants as medicine when given the chance to learn to select.