As far as touristy goes, I'll add that the Ride the Ducks tour is very touristy, and really fun. Lots of Seattle folks wouldn't be caught dead on it, but you get to point at them and laugh while they scurry in the rain so it's all worth it. The tour guides are pretty entertaining, you get to see some cool parts of the city and the whole thing ends by driving into Lake Union and taking a little tour. I went after living here for 2 years and I really enjoyed it.
It's interesting that people prefer not to do the typical tourist things, yet many of those are the most fun. Pike's Place market is a good example. Any sports event (watch Sunday night) will show it with the guys throwing fish. I work near there and in good weather walk up often, because it's a cool place with great deals on local produce. It's also a lot of fun to watch the tourists.
The Ballard locks are also a good place to visit, but just a stroll along the waterfront shops and restaurants is nice as long as you happen to catch a dry day. Snowing here this morning. Some days you will see the cruise ships in port.
If you come in July or August the weather is normally beautiful yet not too hot.
Space Needle, Pike's Place, etc... Also check out some of the small cruises along the waterfront and Lake Union. The one that starts in Ballard and then out through the locks and into downtown was a lot of fun, with a lot of history and things that I didn't know about the area (and I've been here awhile).
Ride the Ducks, Pike Place, Gasworks (paintball scene in early Heat Ledger film), Walk the boardwalk on the sound (shops, Aquarium), catch a ferry to vashon or bainbridge island for a walk or bike ride on the islands (if you are there for 4th of July catch a late ferry back from the island and you'll be able to watch several firework shows from the water), Green River gorge for cliff jumping or laying out rock slabs in a river, City Park at the Space Needle (home of the 1962 world's fair, exhibits, museums, EMP), check to see if there is a festival going on (Bite of Seattle, Bumbershoot, Big Foot, Tug Boat Festival, Hydroplane races), International District, Lake Union Kayak rental, Washington Park Arboretum, etc...
This is a bit more than 5 but there is a lot you can do. Also, you should know that the bus system is excellent, well as far as I remember. Its been a few years. However, they operate on a zone fair system which the entire downtown is a single zone and FREE. So you can go from one side to the other in minutes, like the Space needle to the international district. You can also hop the Mono-Rail out of the city center to downtown.