There's plenty to see in the Emerald City, but if it's your first, second or even third time visiting and you haven't checked Pike Place Market off your list (or the Fremont Troll for that matter), we highly suggest you add both (and the rest of these essential Seattle tourist attractions) to your itinerary.
Pioneer Square
I've lived in Seattle for more than 20 years, and not until recently did I learn that Elliott Bay is deeper than the Space Needle is tall. That's because, like a fool, I waited more than two decades to take Bill Speidel's Underground Tour. Don't do as I did. Go and learn about how Seattle's original streets were once literal rivers of human waste, how a benevolent prostitute created the public education system, and how a lazy immigrant started the fire that burned it all down in 1889. This city, like so many other pre- and postindustrial American conurbations, was founded and framed by thieves and idiots. The tour was created in 1965 by an ambitious publicist who both understood and typified the maxim of the entrepreneurial huckster: If you've got a giant pile of garbage that you can't dispose of, put a sign on it that says "Private Property."
Seattle Center
I suggest you take the 43-second, 520-foot elevator ride to the top of the Space Needle, where you can hang out at the observation deck as long as you like. Go at dusk. If you have money to burn, you may want to eat in the rotating SkyCity Restaurant. (PRO TIP: If it's raining or foggy, it won't be worth it. Best thing to do in that case: Go to spaceneedle.com/webcam, and click "best views" on the upper right corner of your screen.)
Downtown
Less flashy and less expensive than the Space Needle, the observation deck on the 35th floor of the Smith Tower is still very much worth it. This was once the tallest structure west of the Mississippi River. Now with a new speakeasy-type bar so you can enjoy a cocktail while taking in the view.
Waterfront
What beloved Seattle institution is round, white, and 175 feet tall? No, it's not the world's largest Percocet, it's our newest (2012) and most universally approved attraction, the Great Wheel. More like the London Eye than some rickety county fair Ferris wheel, TGW lends a sense of majesty and moment to the waterfront district, granting you an aerial view of the dense buildings rising above the rickety viaduct, the Olympic Mountains, and a glimpse of the Twin Peaks–like mystery of the islands across the water (extra mystery if it's foggy). The price ($14 adults/$12 seniors/$9 kids 3–11) may seem a little steep, but you get three full turns of the wheel, about 15 minutes' worth of new perspective. It's worth it.
Ballard
The fresh water in Lake Washington and Lake Union is about 20 feet higher than the salt water in Puget Sound. More boats travel through these locks than any other locks in the United States, and a million people per year gather round to watch it happen. You can spend a few minutes here, or all day. (PRO TIP: Check out the fish ladder viewing area to watch shiny little salmon swimming for their lives.)
Fremont
Pose for a selfie with the Fremont Troll. (PRO TIP: Better hurry! Rumors of the Troll's imminent demolition have been circulating for years.)
Downtown
There is plenty to see here that doesn't involve people throwing fish—although to be fair, that is totally worth seeing. Fresh flowers, fresh fruit, fresh vegetables, fresh doughnuts—the market has it all. (PRO TIP: At the bottom of the stairs, behind the market proper, there's a great little place called JarrBar.)
Downtown
If you’ve ever wondered what it would be like to work at Amazon (spoiler: LONG hours) or to live in a biodome from a 1970s science-fiction movie (specifically Silent Running), you should go check out these architecturally magnificent, flora-filled orbs that now rest in the heart of downtown Seattle like gigantic toy marbles owned by the world’s richest emperor. Inside tours are open to the public only two Saturdays per month, so you’ll need a reservation, but they’re still mighty impressive when viewed from the outside, or in Understory, the exhibit on the ground floor.
Seattle Center
Firstly, I kind of like the building. I get that I'm in a minority there, but those forceful curves, majestic proportions, and ludicrous colors are so odd and unlikely that they wind up being sort of delightful, especially next to that other people pleaser, the Space Needle. But the inside is what counts, and the inside is filled with interesting nerd matters about rock 'n' roll music (the only major art form that routinely denies being an art form), science fiction, games, and such like. In any other city, MoPOP (formerly EMP) would be a cherished weirdo sanctuary. In Seattle, it's a problem because it was started by Paul Allen, one of our three conspicuous local billionaires. Don't be deterred. If you like that sort of thing, you'll like it a lot. 
