I also have a New Zealand rabbit! They are the sweetest things, aren’t they? How old is yours?
There is a LOT you can do. First, try Googling “diy rabbit toys”. That alone will get you tons of results. Some of my favorites: Take a cardboard toilet paper roll, cut some holes in it (about an inch), stick some hay/a treat in it, put a string through it, and then have it hang from the top of their cage or somewhere where they can grab at it and try to get it down.
You can also take an empty egg carton, fill it with fresh grass, hay, or veggies, close it and poke holes in it (stuff some grass in the holes so they can tell what’s in the carton), and then let them try to figure out how to open it.
With boxes, you can cut doorways in them and put them together so that they form a tunnel. My Dutch rabbit LOVED that when I put it together. My NZW also enjoyed exploring the ones that were big enough for him to fit.
You could also put some yummy things in a paper bag, cut some holes in it, and then let him have it.
A lot of rabbits also enjoy just chewing on cardboard in general. Toilet paper rolls are a good bet. You can also give them an old phone book for them to tear apart. You have a lot of options!
For more ideas, you can go to the Bunny Info section of this website, they’ll have a DIY rabbit toy page. Also, Googling “rabbit boredom busters” is a good idea. Good luck!