You would want to stay in Yosemite Valley - Yosemite Lodge. Or if money's no object - the Ahwahnee. But for August you'd want to book ASAP. Unfortunately the Valley will be very crowded at that time of year so it isn't the very best time of year to visit, but even w/ crowds, Yosemite is a must see. (The high country won't be crowded-but you really want to sleep in the Valley and do the back country as a day trip).

You could also spend a couple of days at Lake Tahoe before crossing the Sacramento Valley to St Helena. Or, if you didn't want more mountains, you could spend those 2 days either in San Francisco or on the Marin/Sonoma coast.

Or you could spend the few days before the wedding in the Napa Valley itself, unless you'll also have time for that after the wedding.

I agree with janisj's recommendations and would emphasize the importance of booking a room ASAP because the most desirable lodging sells out many months in advance, especially in August.

Check out Yosemite Peregrine Lodge. We stayed in a loft there for 3 nights when my parents came to visit. it's about 30 minutes from Yosemite Valley or Wawona.

http://www.yosemiteperegrine.com/

Evergreen Lodge might be considered as if you like hiking tuolomne meadows and Lyle canyon might be worth a day each. The valley in August is worth a walk through on the ring trail stopping at the Ahwahnee for lunch. In August even the must do trails like the Mist and the Yosemite falls trails are more like tours than hikes. I would simply scout out the valley for a return trip in the off season, especially spring. You will come back!