Rental car booths are located throughout the baggage claim area on the ground floor of the Main Terminal.
Should you rent a car during your stay in Seattle? Given a relatively compact downtown and an excellent bus system, on one hand, and expensive parking and bad traffic, on the other, I'm inclined to answer "no".... But the best the region has to offer, like Mt. Rainier and Olympic National Parks, are out of town, driving destinations, and even in town there are worthwhile places that are a bit hard to get to by bus (like Gasworks Park and Ray's Boathouse restaurant).
So my answer would be, "at some point, yes".
Gray Line: Gray Line operates scheduled motorcoaches between the airport and major downtown hotels. Fare is $8.50 one way, $14 round trip. Coaches leave every thirty minutes until 11:20 p.m. There are ticket booths just outside both ends of the Main Terminal Arrivals Area. If you are a solo traveler staying in a downtown hotel, this could be the best balance of economy and convenience.
ShuttleExpress: ShuttleExpress is the door-to-door rideshare van service that operates in most of Seattle and the surrounding area. It's very convenient but not cheap unless you travel in a group ($21 for up to three passengers). ShuttleExpress booth is on the third level of the main parking garage across from the Main Terminal.
Bus: If you are traveling light and want to save money, you can take Metro Bus #194 to downtown Seattle. Buses run every thirty minutes and take thirty minutes to downtown; the bus stop is outside the southern (rightmost) end of the baggage claim area. Note: outbound 194 (which goes to Federal Way) also comes to the same bus stop area; make sure the bus destination is downtown Seattle.
Fare is $2 ($1.25 outside rush hour). In downtown, the bus arrives in the bus tunnel under the Third Avenue during the day (on the Third Ave. itself after 7 p.m. and all day Sunday), and travels as far north as the Convention Center.
Bus #174, a local, would also get you downtown; it takes somewhat longer.
International arrivals at Sea-Tac can be a bit confusing. You go through the Immigration, pick up checked baggage, and go through the Customs. Transit and terminal passengers then part ways; transiters re-check their bags and go through a security check before heading to the next plane. This much is the same as at all U.S. airports.
Passengers who stay in Seattle must also deposit their checked baggage on a conveyor belt before boarding the train to the Main Terminal (no security check). They travel separately from the baggage, which is re-claimed from a carousel in the baggage claim area of the Main Terminal.
It's not a major hassle, but I know it perplexes some travelers (including myself). I have tried to sneak the bags onto the train a few times, but they are pretty strict about it. Presumably this is in order to reduce congestion on the train.
Seattle-Tacoma International Airport (Sea-Tac) is located twenty miles south of downtown Seattle, halfway to Tacoma (hence the name). It is a very good airport in general. From most US/Canadian cities it is at most one transfer away; there are direct flights from Europe (Amsterdam, Copenhagen, London, Moscow), Asia (Seoul, Taipei, Tokyo) and Mexico. Most planes arrive at gates on the Main Terminal - except United, which arrive on the North Satellite, and Northwest, as well as all International Arrivals, which arrive on the South Satellite. Underground trains link the satellites with the Main Terminal.
By plane you'll fly into SeaTac Airport. Take Highway 99 north all the way into downtown Seattle. By car Interstate 5 will get you there if you're travelling from the north (Canada) or south (Oregon, California); Interstate 90 West will get you there from anywhere else.
Unfortuneately there's no metro or subway. Traffic is bad on the freeways. I suggest the bus for public transportation or driving your own car.
Flying into Seatac airport is easy. One of the best airports in the country to get around in. Rental vehicles are right there...no shuttle. Very friendly people and when you walk out of the airport the weather is usually surprisingly cool.. very nice!!
Ok, get a rental car and then drive out of the city and stay out of it. Traffic on the interstates usually is pretty good.
That amazing Mt. Rainier! On a clear day you'll have a good view of it as soon as you land at Sea-Tac.
This airport is the embarcation point for direct flights to Asian AND Europe! On a clear day, you can see Mt Ranier from the terminal.
Flying into or out of Seattle I always request a window seat. On clear days, the views are spectacular. This is Mercer Island, in the middle of Lake Washington.
Delta or United Airlines. If you come from the East, on a clear day, the view of Mount Ranier from the left side of the plane is awesome.
Busses, taxi, walking.