One thing you could do is take a toilet paper tube and put some treats they really like into it, like cheerios, yogies, oats, etc, and then wrap it in one or two sheets of paper and twist the ends of the paper to close up the open ends of the tube, hiding the treats inside (it will look like a giant cylindrical wrapped hard candy). They have to chew through it to get to the nummies which keeps them occupied for a while depending on what kind of chewers they are.
You can also (and this would be a rare treat, just every once in a while) hard boil an egg and give it to them to figure out (rats tend to like egg, so their challenge will be getting through the shell. Don't worry, as a general rule they recognize that the shell is not the part they eat and they just toss it aside. I would still supervise, and if they ditch it because they don't know how to get past the shell, you can crack it a little and they'll start peeling the shell off. This is an activity you'd probably want to do in a safe play space on lots of towels so the clean up isn't as hard.)
I sometimes (about once a month) cut an X in a ping pong ball and push a peanut in the shell inside the ball. I also put a smear of peanut butter or cream cheese on a plastic spoon as a special treat. A dig box filled with paper strips, cloth strips or packing peanuts with a few treats hidden in it is another favorite.
You can put a hardboiled egg or a cob of corn right in the cage if its cage-cleaning night. I have girl rats, and they're a bit nuts so it might be different for your boys, but feather wands, one of those wicker balls with treats stuffed in it, wrapping paper tubes, and all kinds of boxes (especially soda can boxes with a hole cut in the non-opened side) are pretty fun.