How can i make my hamsters cage more interesting.?

I have three hamsters and to keep them from becoming bored I rotate the toys in their cage. Every week when I clean out their cage I put in a new toy or take one out and give them a treat. They love it and for the next week they are in love with it. When I am in the same room as them they bit on the bars of the cage and I think they just want attention. I will take them out and after a while of playing when I put them in they are fine for the rest of the day.