One of my favorite hikes near Ellensburg is the Canyon Hike, just inside the Yakima River canyon on the Canyon River Road. It’s a relatively quick but tough hike, taking about 45 minutes to an hour up, and 30 minutes down.
The hike starts on the Canyon River road, and climbs a ridge to the top, where a sweeping 360 degree view awaits you. You can see the view from my PhotoSynth panorama (requires Silverlight):

At the top is a geocache (the old army ammo cannister near the pole) with a log book inside, a view of the top of Mount Rainier, and views of the Cascade Mountains, the wind farms, the town of Ellensburg, and the Yakima River Canyon.

The hike starts very steep, and is a bit unrelenting, but once you get to the middle of the ridge, the slope eases up, and there are plenty of flat stretches mixed with short climbs/steps up rocky outcrops. The last quarter is steep as well, but rewarding when you get to the top.

Highly recommended if you are in the area!