I'll assume that Red Wing 6 might be more interested in getting a taste of Seattle than in either riding a city bus or chatting up fellow FTers.

First off, you do have enough time for a good excursion, at least to get a taste of the city. Time is your most valuable resource, so budgeting it carefully is a key. Renting a car would not be my first choice, because you'll have issues such as parking and navigating in an unfamiliar city.

So, here is a totally personal set of suggestions for what to do:

• Take a cab from SEA-TAC to my favorite shop in Seattle, The Glasshouse Studio, on a pedestrian street (Occidental Ave) near Pioneer Square. They have beautiful glass pieces on display, and they've also got a glass-blowing works in the shop. I'd do this first, because they close at 5pm. (A taxi will cost about $26-30 from the airport, and will take about 30-50 minutes at that time of day.)
• From the Glasshouse, stroll over a block or two to Pioneer Square. Lots of interesting shops and bars along the way, but most of all you'll get a sense of some of Seattle's street life.
• Head over to Pike Place Market, which is probably the premier tourist spot in Seattle, and deservedly so. It's a collection of food stands and artsy stands and souvenir stands -- a great place for strolling and people-watching. The market closes at 6 and is probably a 10-15 minute walk from Pioneer Sq. (about 8-10 blocks north on 1st Ave), so either hike it or take a cab depending on time/weather/how you feel.
• You'll enter the market at its south end (1st and Pike), and there's a nice park at its north end with a decent view of the harbor (Elliott Bay). If you're hungry, you'll have grabbed something to go from one of the food shops in the market or across the street; for a much nicer sit-down meal, try the Café Campagne, which is the fast-food version of one of Seattle's top restaurants.
• Time now becomes a factor. If you've got the time and energy, checkout the Space Needle and monorail. Walk up Pine Street to Westlake center (at 5th Ave), and take the monorail to the Space Needle. Go up the needle, and take heart in the fact that those people who are in the needle have the benefit of not being able to actually see it.
• The tour is over. If time is tight, cab it back to the airport. If you've got time, you can save $20-25 by taking the Gray Line airport bus from a downtown hotel to the airport.

Happy travels.