Good camping in and around Capitol forest south of Olympia. There's a primitive CG directly to the west of Mima Mounds natural area. From Grand Mound to to Longview the camping is sparce W side, more plentiful E side of I-5. Oregon miles into Portland has 2 legit CGs and plentiful stealth camping, also there's a LARGE island park that's day use only (stealth) as you get closer to the city. And Lief Erickson Drive (stealth) but that's pretty hilly. By the time you approach these, you're about an hour from downtown.
The youth hostel on East bank side- E. Hawthorne? has camping sites in the backyard but reckon you've got Portland accomodations.