someone will know the handy link...
There's a handy link to a list of bunny-friendly veg and weeds - someone will be along shortly who knows it, I'm sure.
If there's a keen gardener near you they may be glad to pass onto you some weeds - dandelions, sow-thistles, groundsel, chick-weed and bobbies-buttons. There are more, too. Our buns especially like the first 3 of these. If people do pass weeds on to you make sure they are still fresh when given - if they've gone off a bit they can cause tummy upsets.
Also any prunings from apple or pear trees.
Fruit-wise buns love apple and pear as treats, but not soft fruit or tomatoes, whereas our guinea-pigs do love strawberries (including the hulls) and tomatoes. But fruits are a bit sugary, so only suitable as small treats.
Any of the brassica family (cabbage, cauliflower, brocoli, spring greens, sprouts and sprout stems) are fine, but be careful in case your buns tums are sensitive to them - increase from small quantities until you are sure, and never give a lot because they are a bit gassy.
Sudden changes in diet are bad for buns.
We find ourselves planning our own veg intake around the buns - combinations that will keep the buns well supplied with our peelings, off-cuts etc. And on holiday it seems unnatural to throw away veg offcuts!
Enjoy.