Seattleites looking for exercise, nature, and quality time with the family come to Seward Park. On any given day, but especially Saturday or Sunday, this place is teeming with people but never feels crowded unless you happen to hit it on a race day. Seward Park is used for triathlons and other race courses because it has a smooth asphalt jogging loop (2.4 miles the whole way around), several gravel swimming beaches, easy access to I-90, and plenty of green lawn for spectators and picnickers. For more information: http://www.ci.seattle.wa.us/parks/parkspaces/sewardpark.htm
This is my favorite place to jog because: 1) the asphalt is smooth and relatively flat all the way around; 2) you get to run right alongside Lake Washington for three-quarters of the loop; and 3) it has mileposts that indicate each half mile so you know your progress. I should note that using the mile markers is a little confusing because the start/end appears to be somewhere in the nether region that is basically the main parking lot. As a result, the first half-mile marker will appear sooner than you think. (On 5K race days, officials use the entire loop as well as add an additional climb up and back down a service road in the interior of the park.)
There are always people walking, jogging, or biking along the loop. Kids love the playground with the slides built into the small hill. I've even seen meditation groups on the beach here. Seward Park is especially popular as a dog-walking spot. There are tons of families who walk their dogs here as a family unit. There are also lots of parents who bring their kids here to ride bikes because even when the kids zoom ahead, they're almost always in sight. In the interior of the park, there are hiking trails. It's a neat way to disappear into "the woods" without leaving the city. Tons of eagles and other birds make the park their home, so it's a great place for birdwatchers.
It's important to note that you should never leave anything valuable in your car. There is a very real problem with break-ins because prowlers know that you'll be away from your car for as long as it takes to do the loop. I've never seen the lurkers, but one time, they broke the window of my in-laws' SUV, which was loaded up with stuff from a road trip, and stole a very nice camera. I come here all the time and have never had my car broken into, but it's got an alarm and I don't keep anything tempting inside.