My husband and I spent a 4-day weekend in Seattle in October and had a fabulous trip. It was very cold (lower 50's) though we only got rained on one morning the entire time and it was a "light" rain. We did lots of research ahead of time on sights we wanted to visit and also transportation options (thanks to the help of tripadvisor). Prior to our trip we had planned which sights/activities we were going to, but we weren't in a time crunch with almost 4 days so we didn't have to schedule a specific daily itinerary. This worked well for us since we were able to choose which sights to see on a particular day depending on the weather, how tired/untired we were, and our mood.
Sights we visited and recommend:
1. Space Needle (of course)
2. Locks boat tour (Argosy line). This was my favorite experience of them all. I've been to visit the Hiram-Chittendam Locks before, but this was my first time going through them. Very interesting. Whether you go on the boat tour or you visit them by land, the Locks are a must-see in seattle!
3. Experience Music Project. This museum right next to space needle is great for music lovers, has a fabulous display of guitars from the 1800's to present including "famous" guitars from your favorite bands, and we loved the hands-on area where you can play/learn to play real electric guitars, base, drums and even make your own song recording.
4. Pike's Place Market (of course).
5. Underground Tour at Pioneer Square. There are many mixed reviews on this tour. We really liked it, though $30 is probably too expensive. But the history of seattle that you learn is SO unique and interesting, which is what makes this tour a "must-see" for me. If you go knowing it is a walking tour and you will be taken to old basements (that are sometimes stinky) then you won't be disappointed.
6. Fremont Bridge Troll. This is not a "must-see," but we were happy we made a quick visit and snapped a photo. It is out of the way from downtown if you don't have a car.
Downtown Seattle is easy to get around without renting a car (plus the bonus is you don't have to pay for expensive parking and you get to walk off all the calories from the yummy seattle food). We were traveling on a budget and didn't mind walking (a couple blocks or up to a mile) to get around to the sights we visited. We used the monorail from seattle center to westlake center a couple times ($2.50 each way), though it isn't a really far distance so you can walk that too. Along with walking we thought the bus system is good in the area. For downtown along the water the waterfront trolley bus line is free, which has a limited area but very easy to use and drops off at some great places (such as Pioneer Square, the Argosy tour boat pier, Ivar's seafood restaurant, and just a block or two from Pike's Place and the Olympic Sculpture Garden). To get from Seatac airport to downtown Seattle we were happy with the bus system (King County Metro express). The ride cost < $3 each way. We waited for the bus <30 mins at the airport and the ride to downtown was only 20-25 mins. I would not recommend using this if you have any tight time constraint between the time you arrive at the airport and when you need to be in downtown; however, for us it was great since we didn't have somewhere specific to be right away. For this trip we didn't make it outside of downtown, though would love to return another time to do so for all the outdoor hikes and activities in that beautiful area.
As for good food, if you like Mexican we HIGHLY recommend Mama's kitchen (on 2nd Ave), a locals favorite (as we were told). For more restaurant recs I would look to other reviewers as we aren't big foodies.
We stayed downtown in two different hotels which we had bid for using Priceline.com. The Hotel Monaco (4 stars) is nice with some great amenities, though the room was very small. My husband and I preferred staying at the 3 1/2 star Renaissance Madison (a Marriott) where we had a king-size bed in a huge room and better amenities than Hotel Monaco. We base our hotel preferences off of the room and in-room amenities rather than other aspects of a hotel such as a gym, restaurant, pool, valet parking, etc.