Seattle is OK for a few days...but it was not as good as I had expected

You should do the obvious things like Pike Place Market, and the Tower. Below the tower are various attractions/museums that make for a full day.
Getting a boat out to islands into Puget sound is supposed to be worthwhile but never tried it

Perhaps like you though I also much prefer outdoor scenic areas. Out to the East round the Grand Coulee Dam and Dry Falls is quite spectacular.....
Actually though in East of the state Spokane I preferred to Seattle. It is also a bit warmer and more reliable wether. Even in summer Seattle can be very much like home for weather/temp

For your best bet though head south a bit

Mount Rainier has some amazing views
Olympic National Park is also quite pleasant (bit rainy though)

Probably best bit for me was Mount St Helens (about 80m south of Seattle). Although it erupted back in the 80's the devastation is still readily apparent. Coming in from East up to spirit lake was great. You can even do Rainier and Mt St Helens in a day for just a Whistlestop tour

Portland in Oregon (its not that far) is better than Seattle (in my opinion). You have the Columbia River gorge with various waterfalls..the Japanese gardens and Rose Gardens are also a good excursion.

A bit further down in Bend Oregon there are some great volcanic features to visit leading to the icing on the cake Crater Lake NP

All in all there is some good scenery to see in the Pacific north west. A circle tour from Seattle could take you through Grand Coulee to Spokane...down from Spokane through Hells Valley Idaho (deeper than Grand Canyon) across to Crater Lakes in S Oregon (def worth a trip) up through Bend to Portland and back to Seattle

Do that and you will have more than enough open space and scenery for any trip...and the mileage is not that bad either! The roads are generally empty

Enjoy