Having just stepped off the plane from LIM / CUZ, I would suggest to skip Lima altogether. With so little time I would concentrate on Cuzco and the Sacred Valley, and maybe visit a couple of other sights (like Pisac or Moray), instead of fighting Lima traffic. Lima's OK, but doesn't have nearly the charm of Cuzco. It takes about 45 mins from Miraflores to LIM, so it's actually a little farther than downtown, where the Sheraton is (note that there currently is a ton of construction right smack in front of it). (Oh, just re-reading your itin, and it looks like you probably can't get into LIM early enough for your flight back to the US...)

In Machu Picchu, if you want to climb Wayna Picchu, you need secure your "permit" as early as possible - they limit it to 200 ppl before 10AM and 200 ppl after 10AM. The view from up there is definitely worth the climb.