All the leafy greens are ok in moderation. Oxalates present in cabbage, kale etc can cause calcium binding too, but they can be given occasionally eg. Once a week.
I used to give mine chicory greens but they don't seem to sell them anymore...could be a seasonal issue.
I also give mine watercress, mustard/salad cress (in the tubs, 25p each), okra, squash, parsnip and carrot (grated) and rocket salad. Bit of fruit as a treat too. Come Springtime mine will mainly be on dandelion leaves and flowers and hawkbit leaves. Just wash them well before offering them up.
The key is variety with beardies.