Spirulina, microalgae, is also a very good choice - with even more benefits!
It does require some extra adjustment for example high temperature (over 20 celsius is good, 35 celsius is optimum) and artificial lights (in winter) but in summer, you can grow it outdoors in open ponds.
Here is couple studies of giving it to rabbit feed (just scroll them as they have also other animals):
I live on the old family farm and have my breeders hanging inside my open chicken coop to make winter months easier for my chores but i built a rabbit tractor and put the litter into the tractor shortly after moving them from the mother and move them every morning before work and evening before dark sometimes ill move them when i get home or mid day on the weekends. They chew the grass and weeds right down to the dead grass and this help cut back on the feed i give them as well. The breeders stay in cages full time but the young i grow out in the tractor its 6x2x1 how ever the next one i will make is going to be taller for the fact the rabbits were crawling out the wire roof since i used a wider wire for part of the top that or a solid top for shade. I fill a small feeder 3/4 full a day for them they eat it and the grass and hay. I also just planted comfery along the house and while it is having a rough grow from the chickens and the cat getting into it, it grow great big leaves that you can harvest and is basically super food. I feed a leaf to my breeders now and again and plan to cut and dry the rest before the cold hits too hard for winter snacks. Dont forget garden scraps look into what they can eat, i just feed a large amount of fresh carrot tops to them even had some left over i dried in the sun for a day and stuffed in the hay rack for them the next.
My rabbits were on forage and grazing only for 8 months of the year. They had a big variety of many of the things listed on this thread and kitchen/ garden scraps, plus grow outs were in grazing tractors. Adults were in colony. Pumpkin, squash, tree clippings, comfrey, dandelion, grass, on and on.