Hay is key to their GI health. GI health is key to their overall health. Boring answer, still true =)
Apart from that, I like the idea about strong colors in vegs and fruit signifying a high content of antioxidants. Dark green vegs, red or dark berries (and the leaves from the plants) like strawberry, bramble, blueberry. I give mine a few pomegranate seeds when I’ve pomegranate myself. (If you have a rabbit with light fur, pomegranate can stain the fur so it look like the bun is bleeding from its mouth, which is scary). I give a few pumpkin-seeds now and then as a treat.
Preventing sickness you do by ensuring good food and good company (bunnies love company!) and good living arrangements (away from draft and moisture f ex), but if a bunny does get sick, it’s so important to detect it early. A bout of gas might be fixed easily at home, but if it progresses undetected to stasis, it’s a vet emergency and it’s really serious. So keep a good eye on the bun, especially appetite-wise. If a bun refuses a treat, it’s an alert. Never wait for the bun to get hungry, the way you can with a(n adult) dog that refuses its food.
Most of this I assume you knew already, but it’s still like the basics. Zeppelin looks adorable in your avatar. Thank you for rescuing him from bad conditions!