How can I keep my gerbils busy?

buy them toys and things to chew on like wooden toys for rodents at the pet store. also having stuff to climb on and inside of is fun for them. things like tunnels and such. a cardboard box might be a bad idea because it will fall apart if they pee in it or chew it. running around a room is only safe if there isnt anywhere where you can lose them or where they can get stuck. wires are also a hazzard. i had a cage and i just took the bottom off of it sometimes and put it outside so that my rats could play in the grass and sniff around.