This is my nightmare lol because I only have 1 kid but I just can't imagine my DS letting me put a baby down for a nap, I don't know how you guys do it with 2+ young ones!! But I did want to suggest quite time boxes -- you can google for examples, but basically you set up 5 boxes (shoe boxes or whatever) and put a few activities in each and then rotate through the boxes, a different one each day so they are always 'new'... You can include a CD if she can work the CD player, a new puzzle, a picture book, toys like lacing cards or snap beads, etc. Maybe even an adult item (cell phone or remote or something) that she's not normally allowed to play with? The whole box should be something she's ONLY allowed to play with during quiet time, so it doesn't lose its novelty.
And set her up with a healthy but fun snack (something she can dip, for example, or fruit skewers) perhaps too.
Just some ideas, I honestly can't imagine it working with my DS so I won't be offended if you say that will never work, but I know it helps some moms!