What level of hikes are you planning? Meaning, are you & the wife in shape go get-em hikers or the mall mosey sort?
I absolutely love Zions, but only spent 3 days there! Camp overnight if you can (September may still be busy, I mean BUSY!) If you want to camp, see about reservations for a campsite NOW. Or stay in a lodge, again, book it SOON. Then take the bus up to ANGELS LANDING and get cracking at that one, then come down, have a bite of lunch that you packed and then bus up & do OBSERVATION POINT. Remember to refill your water. That is a good days worth of hiking and some INCREDIBLE views & experiences!
My wife and I are in between go get-em's & mall mosey variety. It was a hump for a day, but, sitting back at the campsite after both of those, I earned my beers!
We also fit in the 'water falls' meh - its a stupid easy hike with plenty of mouth breathers and not nearly as impressive as ANGELS LANDING or OBSERVATION POINT
I haven't been out in MOAB.
~Will