My kid has this plastic egg toy, like two halves together the kind you get little toys in. Yesterday we were having open playtime and the rats were rolling this around and trying to open it and bite at it and they got really excited rolling it about. They had never seen something like this before so it was quite a novelty to them. They kept swiping it from each other and they even wrestled a bit over who gets to use it.
Rats often tend to play with the most random things. Anything non-toxic can be a toy.
My rats will wrestle and fight with a bamboo chopstick poked through the cage and try to grab it out of my hands and carry it off.