How little is your baby? If he's really young then sadly, there's not a lot of "fun" room with natural decor. You're basically stuck with a boring substrate like paper or shelf liner while they learn to hunt and all that good stuff. But you can have more fun with what you use as caves. You can go to walmart and get packs of plastic bowls in different colors and cut out little openings, or get random objects.
I saw some AMAZING tanks recently that used totally normal things and made super creative themes. (I hope the owner of this tank comes by and links to a photo) Like one had a zombie theme, so he took plastic body parts, hazard stickers, haz-waste containers and stuff like that and make it look really gorey. So you can go to a toy store or a party store and do some really cool and creative things.