I am very familiar with both locations.

If I were you I would skip Tahoe.
My wife and I volunteered at Tahoe for 5+ years and, while it is beautiful, there is not much to do.
There are casinos, a drive around the lake that is in most areas concealed from the road by private homes.
There is a boat ride on the lake and some historic homes.
Not much more.

I would suggest visiting Sequoia National Park, south of Yosemite, to see the world's biggest trees.

As to Yosemite, my wife and son and I have hiked and backpacked Yosemite and we are very familiar with it.
The Tioga Road from Mono Lake on the west to Yosemite valley is a drive that the word spectacular does not do justice.
Steve mentioned things on the valley floor, I would include just riding the shuttle around several times, getting off to view and take picts as you are drawn.

The tunnel entrance is a place you must not miss for its view.
South on 41 about 10 miles is the Glacier Point road, the place where most famous Half-Dome picts are taken.

On this same road is Sentinel Dome and Taft Point.
Taft Point is a flat, easy 5 minute walk.
Stay to the right and see down to the valley floor through fissures in the rock.
You will see a metal fence at the edge, go to the left of that and you can crawl out on an overhang similar to the beak on Half-Dome.

It is breathtaking and you will be looking across toward El Capitan where the climbers hang out but from the height of the top.
I created a Google Map link that shows you the route: https://maps.google.com/maps?saddr=CA-120+W%2FTioga+Rd&daddr=Southside+Dr+to:Glacier+Point+Rd&hl=en&ll=37.707726,-119.505157&spn=0.902844,1.783905&sll=37.714228,-119.595323&sspn=0.014106,0.027874&geocode=FQQKQwId13bm-A%3BFbfaPwIdyGzf-A%3BFd2tPwIdqGjf-A&t=m&dirflg=w&mra=ltm&z=10
and one that shows location of Sentinel Dome: https://maps.google.com/maps?saddr=CA-120+W%2FTioga+Rd&daddr=Southside+Dr+to:Glacier+Point+Rd+to:Glacier+Point+Rd+to:37.71231,-119.603097&hl=en&ll=37.714007,-119.594808&spn=0.014106,0.027874&sll=37.709967,-119.603434&sspn=0.028213,0.055747&geocode=FQQKQwId13bm-A%3BFbfaPwIdyGzf-A%3BFRpuPwIdlT3f-A%3BFd2tPwIdqGjf-A%3B&t=m&dirflg=w&mra=mi&mrsp=4&sz=15&z=16
Good trip.

You could spend a lifetime on Yosemite's Valley Floor and never see it all. To answer though depends on what you want to see. If you have never been before I would suggest you drive in and make the loop as if you were going to leave stopping at all the pullouts along the way, then when you get to camp hop on the first shuttle and take it for a spin.

As to what to see - WOW - on the Valley Floor I would suggest Inspiration Point then down to Bridalveil Falls. Make the short hike in, but be sure to bring a rain cover for your camera if the falls are pumping. From there make the journey towards the campgrounds and the pull outs along the way. Stop across from the little church on your right and take in the Upper and Lower Yosemite Falls on your left. You may even want to take the short walk to the bridge to get a better view. Once going again stop at the meadow across from Curry Village. There are deer there almost everyday. If you pull into the CV Parking Lot there is a magnificent view of Half Dome.

Head to your campground, setup and go for a shuttle bus ride to get your bearings then go for a walk wherever you decide. It will overwhelm you to the point you can sit in the meadow all day long and never feel you wasted the day.

The bridge between the Pines campgrounds across from the stables has an amazing view of Half Dome in one direction and Upper and Lower Falls in the other. Which reminds me, you must see the falls. Again take a rain cover. And if you can do it hike up to Vernal Falls. It is a stretch (the horse trail is easier), but worth it. If you are adventurous start up the trail to Nevada where the steps turn left. There is always a rainbow!

There is so much more, but for now be sure to get the guide they give out for free for any photo walks and enjoy.

Yosemite National Park http://www.nps.gov/yose/index.htm
Photos & Multimedia http://www.nps.gov/yose/photosmultimedia/index.htm

If you have time take the Free Photo Walk. You may know more than they do, but it is sure to be fun!