San Francisco and Seattle are about 679 miles apart by road. Split the difference and you arrive near Portland, the city closest to the halfway point between them. That puts roughly 11 hr 13 min of driving on the San Francisco side and 3 hr 38 min on the Seattle side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so Portland suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Portland doesn't have what you're after, Sacramento and Boise are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between San Francisco and Seattle.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.