From Walla Walla, consider checking out the Blue Mountains. The Blues are incredibly scenic and remote, with a diverse assortment of trees that might surprise you in a "dry" forest. They tend to be steep, with dramatic changes in vegetation from peak to valley, and fun, curvy roads.

Try taking Hwy 12 east and heading up the Tucannon River. If gravel roads without guardrails are ok with you, the best scenic route is to detour off the highway at Dayton, to Patit Rd/Hartsock Grade, then south on Tucannon Rd up the river, getting back to Hwy 12 via Blind Grade/Linville Gulch/Tatman Mtn Rd. Definitely have offline map/music/etc options, cell service cuts out in there.

In between Walla Walla and Clarkston, Dayton is a decent stop. It's small and white with pricey gas, but also a little more affluent and touristy, and they have a progressive-oriented co-op/food park. However I feel like people are staring at me when I stop in Pomeroy, and I'm white and know a couple folks. If you just need a bathroom there are also vault toilet rest stops on Hwy 12 at Dodge and Alpowa; they're generally pretty clean and pleasantly busy.

Enjoy your trip!