Day Hikes In Yosemite
There are several day hikes that are worth doing.

http://www.yosemitetrailsdvd.com/ Great DVD on backpacking and hiking trails all over the park!

http://www.hikehalfdome.com/ Great book on hiking Half Dome from a guy who's done it 20 times!

The Best Time To Visit Yosemite
 

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Thinking of climbing Half Dome?

Read this article appearing in Saturday's (7/7/07) San Francisco Chronicle: Go Here

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#1 Taft Point and Sentinel Dome (1 to 2 miles total)
Go Here for details on a very easy, short hike in the Glacier Point area.  The scenery is awesome, but be VERY careful with small children in those areas where the cliffs are accessible. 

I consider this the number one day-hike in Yosemite due to the easy walk and mind-bending vistas you will witness especially at sunrise or sunset.  Great, great reward for very little exertion.

For a speechless view of the stars, try Sentinel Dome at night.  It's the best 360 unobstructed view of the heavens I've ever seen, especially during the Persied meteor showers in August (8/11 is the peak viewing time.)

#2 Half Dome (Vernal Falls; 1.5 miles and Nevada Falls; 2.5 miles) Go Here
People insist on calling this a day-hike, but I disagree.  It is very long, (8 miles one way to the top of Half Dome) very steep and uphill every step of the way.  But, for those who insist on doing it, here's what you can expect. 

For truly religion-changing vistas atop Half Dome, and for the incredibly demanding and unique physical challenge it entails, I rank this one #2.  It stops short of being #1 due to this trail's extreme physical demands.  I've never spoken with anyone who completed the hike in one day that wasn't totally 100% exhausted when they got back, and vowed never again, and generally regretted going.  It kicked my a** just going from Little Yosemite Valley.
#3 Vernal Falls Go Here
Very popular trail; and for good reason.  It's short, you get within arms length of a gorgeous waterfall, and you get a good cool-down on a hot day! Ridiculously crowded, but worth it.
#4 Yosemite Falls Trail
This trail comes in two distinct sections.  You can go all the way to the top; and yes, the view is fine, but not worth the effort to get all the way.  It's worth it to go to the base of Upper Yosemite Falls, but no further in my opinion.

Due to the ridiculously steep and rocky trail (this upper part of the trail needs work) in the upper section, and the views that are good, but not as good as on the Taft Point trail or from Glacier Point, which you can drive to, this trail rates a 4.
I do not have a page for this hike as I did not take any pictures when I did it... click on the thumbnail version on your left for a full-sized view.
#5 The Panorama Trail Go Here
This is a spectacular trail whose name says it all.  Starting (or ending, depending on how you want to do it,) at Glacier Point, and ending at Happy Isle, nearly every step of the way you get a great vista of the valley.

Yes, the views are great, but it's a long trail, and one you should begin as early as possible. Not because it is difficult, but because you will pause and take in the views frequently.
 
Honorable Mention; The Four Mile Trail

Originally built in 1872 in only 4 months, it was originally 4 miles long, but after the realignment was lengthened to 4.6 miles but retained it's original name.

Views of the valley and Yosemite Falls are spectacular, but it's NOT for those with a fear of heights.  There are stretches where total concentration is required.  I do not have a fear of heights, but even I was very careful in these parts.