SEA is a good candidate for leaving when you use Link light rail and plan around weather. The city is reachable, but Pike Place plus extras can become too much.
What to do with bags
Do not count on public lockers without confirming same-day services. If your carry-on stays with you, pick a route with elevators and simple walking.
Leaving the airport
Downtown, Capitol Hill, or a single waterfront plan can work with enough time. Rain gear and shoe comfort matter more here than travelers expect.
Getting back through security
Build in the walk from the light-rail station to the terminal, then checkpoint and train-to-gate time.