Victoria BC is OK but it takes a lot of time to get there/back. It's on Vancouver Island, so it's a boat trip. Haven't lived in Seattle for 12 years but if there's a direct boat from Seattle, it's 4 hours each way. If you take the Washington State ferry from Anacortes, they used to go to Victoria, and then turn around and go back, so only 1 round trip a day, meaning overnight there. Don't go without a hotel reservation. It's nice, it's quaint, it's British, much like Niagara Falls Ont., without Clifton Hill. And because it's Canada, bring your passport.
Bainbridge Island Ferry is OK but go as a "walk on." There's not much to see there, and if you take a car, you'll pay through the nose.
Mt Ranier is very nice but it's a national park, and a wilderness area. If you're into back country hiklng, it's nice, but it can have temperature/weather swings, so be prepared for that.
Expect rain/drizzle. That's why we left. August & September are beautiful but the other 10 months are drizzle & fog followed by fog & drizzle. Very much a damp marine type climate.
Vancouver BC is prettier than Seattle, but it's a drive up I-5. Instead of the Peace Bridge, they have the Peace Arch, BUT where the peace Bridge has adequate inspection lanes, Blaine WA does not. Be prepared for big delays crossing into Canada there. If you get there, I always felt that the BC ferry system was much nicer than the Washington State system.
The weather is marginal, but there are a lot of great restaurants & bars, so you will enjoy it.