80% of an adult rabbit's diet should be hay. Good quality hay is the most part of a rabbits diet and you're rabbits should Always have access to it. it helps keep their gut moving thus avoiding dangerous gut stasis which left untreated is nearly always fatal.it also keeps their teeth nice and short and in good condition.
Next time I have Guinea Pigs or Rabbits I am gonna have to do this for them. Sisal string, clothespins, and vegetables make for a very happy Bunny! This DIY rabbit toy idea should keep those buns entertained. Only use supervised, though.