Fresh herbs are a good choice. Most rabbits enjoy eating them and they're gentle on the stomach so less likelihood of causing tummy upsets. Basil, coriander, mint, parsley are very popular. As far as I'm aware, the only one to avoid is chives.
Probably best to exercise caution with the brassicas as they can cause gas for some rabbits.
Also, as Shimmer mentions, rabbit safe wild plants are another good source of greens for rabbits if you feel confident foraging. Dandelions are particularly well liked.
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