They also enjoy microwaved or frozen plain spinich, collard greens, dandilions (as long as there is no fertiler or weed killer on them) - pretty much anything green that you or a rabbit could eat.
Briggs can't eat fresh foods - they don't have the mouths for them. So soft foods like fresh cucumber or zucchini work, and so do mushy cooked/frozen. Canas (no longer available in the US) and Marisas (also known as ramshorn snails) will eat EVERYTHING they can find.
Feed 'em well and they'll get to the size of golf balls.