It sounds like you're maybe going to keep the hay on the floor, but just in case you decide not to, would hemp mats like this work?
https://www.zooplus.co.uk/shop/small_pets/hay_and_bedding/hemp_bedding_small_pets/182988 or
https://www.amazon.co.uk/Kerbl-Hemp-Rodent-Mat-100/dp/B00DDC6HPY/ I've had them for Sophie and Casper and they're soft and the bunnies have a good grip on them (I use them to make slippery places less slippery). They're only about 0.5 cm thick, so you could layer some, maybe, to make it nicer for Dennis to lie on for his arthritis. The downside is that if he pees on it you can't clean it, so you'd have to throw it out, and if he digs in it it won't last long. Sophie and Casper can tear one to pieces in a few days.
For toys that are safe to eat... Sophie and Casper have ones similar to these:
https://www.amazon.co.uk/Naturals-Animal-Activity-Banana-Stuffer/dp/B004FGD4AS/ You can remove the "leaves" on top and put treats in them and then the bunnies have fun throwing them around to get the treats out, or chewing their way to them.
These:
https://www.amazon.co.uk/Happy-Pet-Jumbo-Grass-House/dp/B003TL9W7E/ are good as well, because you can stuff them with hay and treats and they can have fun getting it out. Only thing to look out for is if Dennis' head isn't the right size to get stuck in the hole. I can't find the brand I usually have, though, and I don't know if other brands maybe use wire to keep the ball in shape instead of something natural, so maybe you'll need to read some reviews. They're just nests for hamsters, and there are a lot of versions for sale.
This is a fun, edible house:
https://www.amazon.co.uk/Kerbl-Grass-House-Extra-Large/dp/B003YBZITU/ Sophie and Casper love them. It's made of grass woven around some wooden twig thingies, and held together with some natural string. If he'd eat the string, too, that wouldn't be a good idea, though, maybe, though I assume it would just digest.
Willow bridges (
https://www.amazon.co.uk/Trixie-62154-Natural-Living-Willow/dp/B000WFCFG0/) can be fun, too? They're held together with metal wire, but I don't think Dennis would be able to eat so much of the willow branches that he'd be able to get to the wire, and even if he did, the wire isn't sharp or anything, so couldn't do any harm, really. He can have fun stripping the bark off them like he does with his sticks.
Maybe a food tree?
https://www.ebay.co.uk/itm/Trixie-N...-25x36cm-Wood-Rabbit-Guinea-Pig-/163991858187 or a double one for more fun:
https://www.ebay.co.uk/p/27037100146?iid=164096780093 They're wooden, but I don't see any harm in him eating the wood.
These are completely edible as well:
https://www.amazon.co.uk/Naturals-Animal-Activity-Rosewood-Products/dp/B002HEG4ES/ and
https://www.amazon.co.uk/Rosewood-Boredom-Breaker-Animal-Activity/dp/B009EQ7UFE/