Just about any ride up to Skyline is well worth doing - it's a great workout on the way up, beautiful scenery at the top, and an adrenaline rush on the way down.  Some of the routes:

	King's Mountain.  This is probably my favorite ride, since it's a fairly strenuous climb, a steady push all the way up, and very lightly trafficked.  The climb is also in the shade most of the way up, so it's a good bet on a sunny summer day.  Times under 30 minutes are good.
	Old La Honda.  This is a fairly easy climb, shorter, less steep, and with less vertical rise than either Page Mill or King's.  Like King's, it's a steady push all the way up and well shaded, and it also gets very little traffic.  The pavement in good shape, and the top section in particular is brand new.  Times under 25 minutes are good.
	Page Mill.  This is the longest of the Palo Alto area climbs, with occasional steep sections scattered in a mix that even includes some descents on the way up.  It's in the open almost all the way.  The pavement is uneven and many turns are both tight and off-camber, so it's not a great choice for a descent.  It gets some traffic on weekends and in rush hour.  Times under 45 minutes are good.
	Woodside Road (84).  This is probably the easiest of the local climbs, but I literally never go up this way due to the traffic.  It gets a huge amount of traffic at any time of day since it's wider and straighter than the other routes.  Although it's a poor choice for a climb, it's a truly terrific descent - the pavement is new and even, the turns are flat and wide open.  Downhill times under 7 minutes are good.

In addition to the climbs, some of the stretches along Skyline are worth a mention.  The descent between King's Mountain and 84 (after the peak at Skegg's Point) is a ton of fun; the trick is to avoid getting on the brakes and keep your speed above 30 at all times.  The stretch from Page Mill over to 84 is very scenic, with terrific views both towards the bay and towards the Pacific. In the late afternoon the setting sun lends a deep purple shade to the western hills, which are often outlined by a light mist as well.  Spectacular, and well worth the climb.

Here are a couple shots from Skyline.  The first looks east towards Stanford and the bay:

The second looks west towards the Pacific: