You can just give your bun a cardboard box and put some newspaper (both a section of the newspaper, and crumpled-up balls of newspaper). Your bun will dig for HOURS and LOVE shredding up the newspaper. I know mine do!
Mine prefers to dig newspaper in large cat litter boxes. (Go figure!) Sometimes it's a matter of finding out what they like - they *do* have minds of their own, and distinct preferences re. toys and other things.
I give my girls boxes full of newspaper (they shred it them dig it out), towels to dig at and boxes/ plant pots full of soil (definitely the favourite but a bit messy! :rofl
Just make sure the soil doesn't have any pesticides or fertilizer in it, and if you use any grass seeds to grow grass, make sure they're untreated, and you should be fine!