Snoqualmie River: Fall City to Tolt River Mouth is a 11 mile trail located near Fall City, WA. The trail is primarily used for canoeing.
Floating the Snoqualmie River is one the most joyous experiences you'll have if you do it on a hot day with good friends. I organized a river float with 12 friends back in the summer of 2010. While we were on it, one of my friends said it was one of the most peaceful experiences he'd ever had. It's difficult to overstate how much fun this experience is.
You can rent tubes but I recommend just buying your own. Tubes can be had at 18 wheeler truck supply stores in South Seattle (google a bit). Buy one small raft to hold your cooler of beer. There are a couple rope swings you cna hit on your way down.
Be careful where you put in as there are sometimes rangers writing tickets for not having enough life jackets. Also be careful of how much alcohol people consume (as you are on a river).
Most importantly, enjoy.