I think the pp may have been referring to St Edwards State Park - it immediately came to mind for me too - it is beautiful and has an AMAZING playground that is fun for a wide range of ages. Also lots of open grassy areas, trails, etc. It is on the north shore of Lake Washington, in the Juanita/Kenmore area.
Edmonds City Park is much smaller, but really nice for events like reunions, with big playgrounds, a baseball field, lots of grassy space and trees, and a wading pool. It is also close to several beach parks and the ferry terminal. We live very close to it, and see big family reunions there on a regular basis throughout the summer.