I have two ferrets, a young-ish male (Max) and an elderly female (Tilly). Unfortunately, as a full-time university student I don't have much time to take them out of the cage anymore. My house does not allow for free roaming so we need to block off the main hallway whenever we take them out of the cage. Also, due to some unfortunate dryer accidents both of their hammocks are broken. I also had to take the bed out of the cage because Tilly wouldn't stop trying to eat it (she was pulling off massive amounts of fibres). Needless to say, the cage is basically back to its bare bones...

They have:
-2 towels
-1 hanging crinkle tube
-1 cardboard box
-1 hanging parrot toy (that they don't play with)
-1 bell ball (that they don't play with)

The entire bottom half of the cage is litter boxes because someone *coughMaxcough* prefers to use the cage floor.

I'll try to get a new hammock but my mom might insist on fixing the broken ones and then never do it.

What kind of items can I put in the cage to make it more interesting for them in there?

How about purchasing a play pen for them. Give them more room to play.

https://www.ferret.com/item/marshall-ferret-playpen-11-panel/650889/

Mat that goes on bottom  and/or top

https://www.ferret.com/item/marshall-small-animal-playpen-mat-cover/650280/



* dry hose that they can run thru or you can use pair of pants.

Have you tried attaching cat teasers to the inside of their housing? I attach cat teasers to the ferret enclosure when I find good strong ones, just weave the car teaser handle/stick in between the bars to attach it. 
That reminds me, I need to get a new one, Bandit killed the last one, quite literally, chewed her way into where the jingle ball is then broke the ball.

We just got a smaller play pen that they can use for shorter play sessions but they can easily get through the bars. (Tilly doesn't even need to squeeze. She just walks right out.) We're looking into fixing some plastic sheets to it. 

They don't like bells or balls so those won't help at all. I did pick up some new crinkly toys (their favourite) today so at least they'll have something new to play with in their next session. If they really like them I might put one in the cage for them.