It's official title is the Union Bay Natural Area, but locals know this stretch of nature as the Montlake Fill, and it was my pick for the best morning walk in the city in Seattle Before8.
Amazingly, the Montlake Fill used to be a garbage dump, before the city capped the garbage in clay and began fostering plant growth over it. Since then, it has become one of the premier birdwatching (or birding) locations in the city, with hundreds of species filling its skies at all times of the year.
While pleasant to walk or jog through alone, a guided tour is almost necessary if you want to experience all of the Fill's little secrets. The Seattle Audubon Society puts on free walks every weekend, and they are a must-attend if you're visiting the city.