My 5 1/2 year old is just starting to go off naps (sometimes). He probably naps 5 days a week (he needs it!). On the days that I can tell he just doesn't want to, I give him books from the library (we have "story books" to read, but then I'll also get something bigger: a human body and how it works book that has interesting pictures from the bigger kids section of the library, OR a neat photo book that can keep him looking and thinking...colors or a pen and some paper, or permission to write on his dry erase board and draw pictures, write his name, numbers, or practice letters. Those magnetic letters to make words? Little green army guys that take up time to set up and make little battles in your mind? Another thing I do sometimes is take the kids swimming or racing, or just running a few laps and then stretch, and then cooling down with a drink of water or lemonade before nap helps. (Joseph's class runs laps around the gym for kung fu, so he doesn't have a problem with me telling him to run along the fence line 3 times). When I used to provide inhome childcare the little kids took naps after snacktime, which was after I picked up a 6 year old from half day kindergarten. So, I'd pick up the 6 year old, we'd all have snack and chat, and after snack little ones went to bed and the older one could watch a movie, play a board game or cards with me, play doh, do his homework, or do a special craft that I couldn't easily do with little ones around as much (paint, watercolors, instead of reading a story to him which is what I'd do if everyone was awake, I'd work with him a little on sounding some words out and "reading" together or helping with counting beads, pennies, whatever while the little ones slept...etc). Sometimes we'd do little projects where he thought he was helping....we'd sort the money in the change jar and learn what each coin was, and I let him keep the money that was the year he was born. After sorting the coins, we'd count them out and roll them. It sounds stupid, but it was fun for the little boy and just reenforcing what he was learning at school. It's one thing to look at a picture of a quarter in a workbook and another thing to hold the quarters. Babysitting takes creativity. And in my experience, active children need more rest. I might make someone mad, but I am guessing that if a 2 or 3 year old is not napping, then they need to watch what they're eating, and am guessing that they're not swimming or running around at the park a lot. Just saying...
On top of suggesting things they can do and providing materials (the ds, books, etc) I would suggest that the nappers be in one room and the non-nappers lay down on their mats in the living room or den. That way they won't "disturb" the nappers so much. How about learning how to do different paper airplanes and sending over a bunch of paper? (I had the Dangerous Book for Boys and learned a few airplane styles and the 6 year old I babysat loved to make fleets of planes and then to make it last longer I'd give him crayons and have him decorate the planes. I made mine with a pilot waving in the window, and he got excited about that and worked hard on his "fleet").