Stroll around the iconic pike place market! Get there early to avoid huge crowds.
Get tickets ahead of time to go up to the top of the space needle! Would also recommend looking into getting the City Pass if you are planning to see other attractions such as the Chihuly Glass Garden, Aquarium, and a few others.
Rent a car for the day and drive to North Bend for this 4-mile roundtrip hike with killer views! Get there early to avoid crowds.
Take the water taxi from downtown Seattle to West Seattle and get lunch at Marination! There's also a lovely walking/biking trail along the water with sweeping views of downtown Seattle.