How far are you willing to drive? North Park is the only course that's actually in Seattle, but not the best experience. SeaTac is a little south of the city but a great course, and if you are down there and 27 holes wasn't enough, Lakewood is only a short drive away. Terrace Creek is nice and relatively close to I-5 north of the city but can be really swampy in parts depending on rain. Tall Firs is great, as mentioned, and costs $5 to play.
SeaTac, Lakewood, and Tall Firs would be my recommendations. If you have an extra day and are willing to drive south, I'd check out Fort Steilacoom (two distinct 18-hole courses and a 9-hole course.)