The park has so much to offer.
I'd say to definitely go to Glacier Point. If you take Highway 41, you will pass the turn-off to Glacier Point on the way to the Valley. Right before you get to Glacier Point (like a half mile, maybe?) There is another point called Washburn Point. Stop there first for a view of Half Dome & three waterfalls - Vernal, Nevada, & Illouette. Then from Glacier Point again, you can see Vernal, Nevada, and around the point you can see Yosemite Falls. The valley will be spread below you. It is a spectacular sight. There is a little hut there that has maps on the wall that illustrate how Yosemite was formed, and you can get some appreciation of the magnitude of the grandeur that is Yosemite.
Once you visit Glacier Point, you can go down to the valley. You will go through Wawona Tunnel and right on the other side, when Yosemite is unveiled before you, there is a perfect spot - Inspiration Point - where you can stop & see another view of the valley, seeing Bridalveil Falls, Sentinel Dome, Cathedral Rocks, Half Dome, Clouds Rest, El Capitan, and many other points to see.
When you go down to the valley, you will pass a little parking lot for Bridalveil Falls - stop there for a very short walk to the base of the waterfall. Down in the valley you can also drive to a parking lot and walk to the base of Yosemite Falls. I also recommend visiting the Visitors Center there, and perhaps stopping at Curry Village & having a pizza there at their outdoor deck.
This should really take most of your day, but if you just want to touch these places & see the most you can, then when you leave, going on Highway 41 again, pass the entrance to the park, and go another two miles to the Mariposa Grove of giant sequoias.
I have a couple of links in my signature w/pics of Yosemite in them.