This is about a half mile round trip to the Old Tree, a 12 foot diameter, 297 foot tall redwood. There are several interpretive signs along the way.
Since GPS coverage is somewhat spotty under the redwoods, here is some additional info: about 90 feet from interpretive sign, at a bearing of 130 degrees magnetic.
Update on 6/7/2012: The latest measurements of Old Tree, provided by yofoghorn:
It is 305.38' tall and 14.61' wide. This height was measured to the inch with an Impulse 200 laser transit and tripods. It is the most accurate measurement of the tree. And the width was measured with tape. Also, this tree is special as it is only one of seven redwoods south of the Golden Gate over 300' tall, three of which are in Portola. One of the others is near Slate Creek and the other is in Peters Creek Grove. Old Tree is the tallest in the park, but is beat by two trees in Big Basin. The tree is still growing in height, at a rate of about two feet every ten years.