Hiding treats is good. You can also try a rolling treat dispenser or dangle an apple bit from a string. If you hang some fabric down one side of the cage they may enjoy climbing it and tearing it up. Some rats enjoy a rat-approved wheel (large enough with solid footing, not wire mesh). Then again, I've found that many rats tire quickly of the wheel.
Of course nothing can replace free-roam time...when they're ready and you are ready