Though Seattle's not always the easiest city to navigate, it's small enough that you can see a great deal of it in a week. If you've only got a long weekend here, you can easily mix and match any of the days in this itinerary. Before you explore, you'll need three things: comfortable walking shoes, layered clothing, and a flexible mind-set: it's easy and advisable to meander off-track.
Spend the first day seeing some of the major sights around Downtown. Get up early and stroll to Pike Place Market. Grab a latte or have a hearty breakfast at a café, then spend the morning wandering through the fish, fruit, flower, and crafts stalls. When you've had your fill, head a bit south to the Seattle Art Museum or take the steps down to the docks and visit the Seattle Aquarium. Other options include taking a stroll to Belltown to take in the views at the Olympic Sculpture Park or getting a roving view of the landscape from atop the new Seattle Great Wheel Ferris wheel. Stop for a simple lunch at El Puerco Lloron or Macrina Bakery. If you're not too tired, head to 1st Avenue in Belltown or to Downtown's Nordstrom and thereabouts for some late-afternoon shopping. Have dinner and drinks in either Belltown or Downtown—both have terrific restaurants that will give you a first taste of that famous Pacific Northwest cuisine.
Take the two-minute monorail ride from Downtown's Westlake Center to the Seattle Center. Travel up the Space Needle for 360-degree city views. Then take in one of Seattle Center's many ground-level attractions: the Pacific Science Center, the Children's Museum, the new Chihuly Garden and Glass exhibit, or the Experience Music Project/Science Fiction Museum. If you didn't visit it the day before, walk southwest down Broad Street to the Olympic Sculpture Park. From there, take a cab or the bus to the International District. Visit the Uwajimaya superstore, stroll the streets, and have dinner in one of the neighborhood's many restaurants.
OPTION: If you don't need to start out with the Space Needle, skip Seattle Center and start your day in Pioneer Square. Tour a few galleries (most of which open late-morning), peek into some shops, and then head to nearby International District for more exploring—don't miss the Wing Luke Museum of the Asian Pacific American Experience. If you still want to see the Space Needle, you can go after dinner; the observation deck is open until 11 pm most nights.
Now that you've seen a little bit of the city, it's time to get out of town and get closer to nature. Start the day early and get out there. Hikers almost have too many options, but Mt. Rainier National Park never disappoints. Plan a whole day for any hiking excursion—between hiking time and driving time, you'll probably need it. When you return to the city, tired and probably ravenous, grab a hearty, casual meal and maybe an art flick—Capitol Hill is a great neighborhood for both, as is Wallingford and the University District.
If you'd rather take to the water, get on a ferry and check out either Bainbridge or Vashon islands. Bainbridge is more developed, but it's pretty and has large swaths of protected land with trails. The Bloedel Reserve is a major attraction, with trails passing through a bird refuge, second-growth forest, and themed gardens (a Japanese garden and a moss garden are just two). It's always serene, thanks to a limit on the number of daily visitors (make reservations). Vashon is decidedly more agricultural and low-key. The most popular way to explore either island is by bicycle, though note that Bainbridge has some hills. Both islands have beach strolls, too. Bainbridge also has many shops and good restaurants, so it's easy to grab a bite before heading back into the city. You'll need less time to explore the islands than you'll need to do a hiking excursion, so you can probably see one or two sights Downtown before going to the pier. If you haven't made it to the aquarium yet, its proximity to the ferry makes it a great option.
Since you covered Downtown on Days 1 and 2, today you can sleep in a bit and explore some of the different residential neighborhoods. Check out Capitol Hill for great shopping, strolling, café culture, and people-watching. Or head north of the Lake Washington Ship Canal to Fremont and Ballard. Wherever you end up, you can start your day by having a leisurely breakfast or getting a coffee fix at an independent coffee shop. To stretch your legs, make the rounds at Volunteer Park in Capitol Hill or follow the Burke-Gilman Trail from Fremont Center to Gasworks Park. Both the Woodland Park Zoo (slightly north of Fremont) and the Hiram M. Chittenden Locks ("Ballard" Locks) are captivating. In Capitol Hill or in the northern neighborhoods, you'll have no problem rounding out the day by ducking into shops and grabbing a great meal. If you're looking for late-night entertainment, you'll find plenty of nightlife options in both areas, too.
Spend at least half of your last day in Seattle outdoors, exploring Discovery Park or renting kayaks in the University District, from Agua Verde Café and Paddle Club, for a trip around Portage Bay or into Lake Washington. Linger in your favorite neighborhood (you'll have one by now). Note that you can combine a park visit with kayaking if you head to the Washington Park Arboretum and Japanese Garden first. From there, it's a quick trip to the U-District.
Pike Place Market and waterfront attractions are open daily all year.
The ride to the top of the Needle is not worth the charge if the day is overcast or rainy.
Remember that Pioneer Square's galleries may not be open on Monday. They do, however, have late hours on the first Thursday of every month.
When leaving the city for an all-day hiking trip, try to time things so that you're not hitting I–5 during evening rush hour. Unless you get a very early start—or don't leave until 9:30 am or 10 am—you probably won't be able to avoid sitting in some morning rush-hour traffic.