Summer is almost over <sob> but a great way to cool off during the last warm days of the year is to get out on Lake Union or Lake Washington. I rented an SUP (Stand Up Paddle board) from Northwest Outdoor Center on Lake Union for an hour and I had a great time. Paddle boarding is a new sport where you stand on a surf board and paddle with an oar to move yourself around. I didn't take a lesson because it's not too difficult to learn. If you know how to use an oar, you can paddle board. The first time I tried it I struggled for about 20 minutes with balancing on my feet on a more advanced board, but NWOC has 3 different kinds of paddle boards which range from easy to expert, and the easy boards are very stable. I watched the following video by John Denny and East Coast Paddle Surfing before I went out on the water and that was all I needed to know.
It only costs $13/hour plus tax to rent a paddle board and oar, and NWOC provides you with a life vest. I recommend calling ahead to reserve your paddle board to make sure they will have one available to use.
The view from Lake Union is spectacular, of course, because you have a great view of Gasworks Park, the Space Needle, St. Marks Cathedral, downtown Seattle, and more. My only complaint was the noisy sea planes, but it is kind of cool to watch them take off and land in the lake.