Your setup is looking great!
Like Isadora said, don't use cat litter since it contains clay which can be fatal if ingested. You can use paper hamster and rabbit bedding, or you can use wood pellets.
@Kiki uses pelleted pine litter and it's been working great.
Cat scratchers, cat tunnels, cardboard boxes, etc. are all great enrichment. Toy can be made out of cardboard rolls and tubs, and you can stuff them with hay. Here's some videos on how to make some
I've also cut and braided paper bags to make toys out of, and using cardboard scraps I've put them on a string of twine and rabbits love them. I'll see if I can find some pictures. They were like this
Rabbits also love pinecones, but be sure to put them in boiling water to rid of any harmful insects or parasites that could be in it.
Willow and apple wood sticks also make great chews and can be purchased from Amazon, petco, or Small Pet Select.
Switch out and rotate the toys every so often to keep it new and exciting for them. That way they won't become board of them easily and you won't have to make or buy new toys all the time.