Ride the Ducks Seattle. A 90-min ride on an amphibious World War II vehicle (yes, part of the ride is on Lake Union), not cheap ($23 adult) and not for those with a limited sense of humor (the style is a bit over-the-top). Definitely unique. 5th Ave. and Broad St., across from the Space Needle. Open-year round.
Argosy Cruises. Offers sightseeing cruises of the harbor, the locks, and the surrounding lakes.
Seattle Underground Tour. will take you underground in Pioneer Square. In 1889, 25 square blocks of Seattle were destroyed in a fire. When rebuilding, the city decided to raise the streets in the city approximately one story. Eventually the sidewalks were raised as well, and people traveled between the second story of the rebuilt buildings. The Seattle Underground was born!
The Great Pacific Northwest Brewery Tour Company. Tour several of Seattle's best breweries. Tours last about 3 hours and include transportation to and from 3 breweries, tasters, score card, and a souvenir glass. You might even get to meet the Head Brew-Master.

Museums
Seattle is home to a number of top-notch museums. Downtown is home to the renowned Seattle Art Museum, which displays an good overview and assortment of art from around the world. In the Central District is the Seattle Asian Art Museum, an off-shoot of the Seattle Art Museum which focuses on Chinese & Japanese Art, but includes works as far away as India. Nearby is the Frye Art Museum, a small private collection featuring 232 paintings by Munich-based artists.
Surrounding the Space Needle on the grounds of the Seattle Center are several more big museums, including the Pacific Science Center, an interactive science museum, the Experience Music Project, a Rock & Roll museum with a special Jimi Hendrix exhibit, and the Science Fiction Museum Home of the Science Fiction Hall of Fame.
Downtown is home to the popular Seattle Aquarium. The University District holds the The Henry Art Gallery, one of the biggest contemporary art galleries in Washington. Also on the university campus is the Burke Museum, a combination natural history/archaeology museum. Further out in Georgetown is the Museum of Flight, with a large collection of aircraft ranging from wood and fabric crates to the sleek Concorde.
Travelers planning to visit multiple attractions may benefit from Seattle CityPASS, which grants admission to 6 Seattle attractions within 9 days of first use for a much reduced rate and includes expedited entry in some cases. The included attractions are: Space Needle; Seattle Aquarium; Argosy Cruises Harbor Tour; Pacific Science Center; Woodland Park Zoo and an Option Ticket with choice of either Museum of Flight or Experience Music Project - Science Fiction Museum.