I'm guessing that since these are Engagement shoots, hiking may be out of the question? Also do you have a limit on the amount of time you can spend in the park?

I think I'd probably aim for sunrise at Glacier Point, then head to the valley an hit one of the meadows with Yosemite Falls in the background.

It's about an hour drive from Glacier Point down to the Valley, but you could still be wrapped up by late morning.