A fun thing you can do with those balls is zip tie them in halves to the sides of your cage as a sleeping spot. Most rats get claustrophobic and stressed out when you put them in one of these thinking it'd be good for some physical stimulation but it makes for a retro looking nesting spot. If you want, you can buy them a 12' wheel (Wodent Wheel and Silent Spinners are some safe, popular choices) and they can run on that for fun.
Some smaller rats seem to enjoy a large rat sized exercise ball, but to be honest they are better off free ranging and being able to sniff and investigate better (unless theres an area thats not rat safe, but a ball isnt a replacement for free range). I would let the rat choose if you do use one though, let it go in of its own accord, then close and put on the floor. if the rat seems distressed remove it straight away, if they start moving around let them. The one rat i've owned that actually liked running in a ball was trained to run into my leg when he wanted to get out. He liked a quick race around then to come back out, mainly he seemed to enjoy crashing into things (he was a teenager lol) then coming back looking all proud of himself. Really though they make better beds for the cage.