Keep in mind that Seattle and Tacoma are a fair distance apart.
Tacoma has the Chihuly Glass Museum - that's the building down by the waterfront that looks like it's got a giant Apollo capsule tipped over a bit on one end. There's also Point Defiance, which includes an outdoor museum area replicating life during the pioneer days, as well as a respectable zoo and aquarium. There's a free trolley that runs from the train station, past the domed stadium and the University of Washington Tacoma campus, on up to the theater district - don't know how much longer it'll be free, though, because Sound Transit is getting desperate for a way to raise money to improve rail transit around these parts.
Tacoma does boast a number of good restaurants, none of which I've been to because I prefer diners (Stanley & Seafort's has a beautiful view, but I bet they won't serve me eggs over hard and ham fried in butter at dinnertime!). On the other end of the culinary scale, there's also the Brown & Hailey factory, where they make Almond Roca, Mountain Bars, and other such delicacies; the factory also has a store where you can get good deals on factory seconds.
If you do decide to go to Seattle, you'll want to park somewhere outside the city and take public transit in; the mayor seems dead-set on forcing everyone in Seattle to abandon their cars, whether they want to or not, so parking is getting very hard to find and the price rises frequently. The Space Needle is part of the Seattle Center complex, created for the 1962 World's Fair; the complex also features the Pacific Science Museum, with displays that change frequently (although I believe they still have the Gemini capsule that you can climb inside of, and of course the giant billboard-like thing with the plexiglass and rubber balls, illustrating Poisson distribution).
There are a number of things to do in either Seattle or Tacoma (or in the cities in between), depending on your interests...
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Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.