Mount Walker is a nice drive-up or summit hike just off Highway 101 on the east side of the Olympic Peninsula. There is a good view of Puget Sound from the summit and thus it makes for a good spot to try your hand at VHF/UHF contacts.
Applegate Peak is another summit in Crater Lake National Park, but has more challenging route-finding and hiking than others. It is about 1.7 miles one-way cross-country - there is no trail or significant landmarks. A GPS with waypoints and a feeling for following the 'easy way' up draws and drainages will be useful for finding Applegate Peak, and more importantly, you way back.
Highway 12 between Morton and Yakima is out of the way, but a scenic delight. If you find yourself driving through, this is a nice summit to activate. There are a number of other summits in this area - several of which appear to require significant rock climbing skills but this one is a straightforward hike.
My general guidelines for off trail hiking are a mile or so across open sagebrush steppe, perhaps half a mile through open forests like ponderosa pine, and about a quarter mile for a western Douglas fir forest. This activation is a reminder to stay with these guidelines and not ever wander through a lava flow again.
I started from the Carson Guler Road, parking about 2.5 miles east of the gate for Red Mountain (W7W/LC-036 - a recommended activation).
In August 2013 the supervisor of Crater Lake National Park closed the Llao Rock Research Natural Area - including the summit of Llao Rock - except when snow-covered (during winter, when the park roads are closed). When visiting in July 2014 a sign was found describing the closure (see photo).
Stevens Peak in Mount Rainier National Park, is a committed alpine climb. It's recommended you prepare carefully reading guidebooks and climbing websites, and making sure you have the necessary skills, experience and equipment. You will need to be able to handle wilderness route finding, high angle snow ascent and descent, mixed rock and snow, rock climbing, and challenging terrain.
There are two Unicorn Peaks in Washington - this Unicorn Peak is in the Tatoosh Range in Mount Rainier National Park. Unicorn Peak is a committed alpine climb - it can't really be considered a hike.
Here's another four-point summit north of Carson, Washington. You might be able to get within a mile or so in a carefully driven passenger car - perhaps only a little farther in a high-clearance vehicle.
There are a nice bunch of summits north of Carson, Washington - this one is a near drive up - as long as you have a high clearance vehicle.
The summit proper of Mount Dallas on San Juan Island, Washington is privately owned and not accessible.