My suggestion might not be the most "natural" but it is what I did when my then four year old quit napping ( Before that I would nurse dd while patting ds' back and have them go down for a nap at the same time...blissful!).
When he quit napping regularly I.....put on a tv show for him
ing. He doesn't watch a whole lot of tv and I let him watch anything he wanted ( within reason) while I put dd down for a nap. He was told to not bother me during this time, and if for some reason there was an emergency he was to sneak into the room, quiet as a mouse and motion to me, and NOT TALK!!! I then explained that if he watched the show and didn't bug me and I got dd down for a nap I would do something special with him. He understood that if I couldn't put dd down, we wouldn't be able to have our *special time* together. This went the same for being loud and waking her up, which then I would have to go in and settle her down. I'm pleased to say that it has worked and continues to work now that he is 5. Yes, he watches a 30 min. cartoon every day, but then we play a board game, do some art, play, whatever every day too, alone. And he loves the individual attention.