Pacific Science Center, Seattle (must see)

The first U.S. museum founded as a science and technology center, the Pacific Science Center aims at advancing public knowledge and interest in science. Designed by Minoru Yamasaki, who also designed the World Trade Center in New York, its five buildings contain interactive exhibits, two IMAX theaters, the Butterfly House, an excellent planetarium, and laser shows that feature music synced up to lasers.

Hands-on math and basic science exhibits delight school-age children, and other exhibits excite the inquiring mind with demonstrations of virtual reality, computer science, and robotics. Two of the many exciting permanent exhibits are 'Dinosaurs: A Journey Through Time', which features eight full- and half-size robotic dinosaurs that roar; and the 'Insect Village', inhabited by live and robotic insects, and a beehive.

Why You Should Visit:
One of the better science centers in the US considering the variety of options available and the level of detail.
They offer different plans, either for the exhibits alone or with the movies (plenty of movie offerings, too).
The gift shop is very nice and full of many well-priced items for taking home as gifts.

Tip:
If looking to do lots of things downtown, make sure you take advantage of the CityPass as it will save you 40-50% and you can knock out lots of attractions.