Cardboard.
Nothing to worry about, yes, but not in excess amounts.
There's rabbits who will just nibble on or spit out the parts they've ripped off, and then there's those bunnies who will literally run away from you with a chunk of box to eat it somewhere quiet. That's what 75% of my rabbits have been, and currently cardboard toys in our place is a no, because mine know no limits and would gladly eat until stomach blockage.
The only exception i would consider for us is something akin to those cardboard cat scratchers without any adhesives or catnip, those can be made at home, small ones in stores are about 5€, if you just happen to have excess boxes or can get hands on some. Stores and restaurants on delivery days are a good option to aim for.
Mine also love fresh twigs from our yard, especially apple, and last spring Storm had a blast with a thicket of cut willows; like literally just a buuunch of big twigs tied into a bush. He wouldn't even try the ones from the store.
The plastic cat ball with bell was something everybun played with as well, even more after i wove some thicker jute through the gaps, until they tossed it somewhere it got lost.
One other random thing would be just plain plastic boxes. They don't even have to be new or empty. My folks love to hop on them and nudge them around. It might actually be worth a try to make a fort out of those...
Or if you have some tools and will to diy, and a sawmill (or other source of wood you know would be safe) nearby enough, maybe gather some supplies and build a custom castle that could be more fitting to your bun's size and the layout of the house?
I don't know how your situation is, but i find it extremely hard to get toys, especially tunnels that would fit five kilos of french lop, and then some more for his medium gal.
Blanket forts are also a good idea, if your bun won't wreck those, and i have often found decent fleeces for a few euros in second hand stores. Just wash before use and that should do the trick.
Good luck in your enrichening endeavors!
