Couple things I do to entertain them is play hide/seek. Show them a toy/treat, then I put them in a room where they can't see me. I hide the toy/treat somewhere in the house and then let them out. They absolutely LOVE this game.
Another thing I do is to get a small blanket (really cheap one) and put a toy/treat under it. Zoey then "digs" to get the toy/treat but it takes a while since it gets bundled up. With Magnus, he's bigger so I make it harder by tying the blanket so he'd have to untie it to get to the toy/treat.
Here's a youtube clip of what I described.
Digging Indoor! (Magnus) - YouTube Digging Indoors! (Zoey) - YouTube