We heard about Golden Garden Park in Seattle through a friend (http://www.seattle.gov/parks/park_detail.asp?ID=243). This park is very good for families in my opinion. We can see sunset, play in the beach sand, very good children play area (new playground in Aug 2011), many fire pits, etc. If you are small family like us then one of the portable propane stoves in http://www.coleman.com/coleman/ColemanCom/subcategory.asp?CategoryID=2010 will come very handy. Yelp comments in http://www.yelp.com/biz/golden-gardens-park-seattle gives a good overview about this place. When you are parking in boat lauch parking spots, better watch for the rules though the facility for paying fines are available online, you can avoid if you take care of the rules.
Green Lake Part is also an awesome place to hang around in summer. It has a rental boat center (http://www.yelp.com/biz/greenlake-boat-rentals-seattle) and lots of place to walk, jog and play. http://www.seattle.gov/parks/park_detail.asp?ID=307 is the park website. http://www.yelp.com/biz/green-lake-park-seattle has a very good review.
When you get some time, plan to visit Ballard Locks (http://www.nws.usace.army.mil/PublicMenu/Menu.cfm?sitename=lwsc&pagename=mainpage). Lake Washington is above the sea level. So, to connect Lake and Sea, Ballard locks helps. If you are new to this concept, then it is worth a visit.