Take your time and bring your camera to this wonderful Seattle area attraction. While a bit costly, if you get this as part of a discount ticket package - add it on. If you're going to Tacoma, see the bridge of glass there instead.
Very interesting historical adaptation of glass, vibrant color gardens and amazing opportunity for photographers. This is something to do both on a sunny or rainy day. Takes about an hour unless you want to read everything and take tons of pictures.
I went midweek during the day and there were no crowd. Great cafe next door for lunch or a beverage break during a busy city visit.