Lynden Sculpture Gardens isn't really an official park, but it is a lovely green space where you can get outdoors and have some fun. The downside is that you have to pay to get in because it's privately funded. It might be worth your while if you have an interest in outdoor art or want to participate in one of their hosted activities, which can include speakers, classes, or workshops both indoors and out.
I've been there in all the seasons, and my favorite time to go is in Fall or Winter. During the Fall, you get to see the reflected colors of their many hardwood trees in the pond. During winter, you can strap on your cross country skis or snow shoes and take a walk around the grounds. There's a main building where you can go to warm up or use a bathroom.
I can't actually include photos of the sculptures because they are protected images, but I have included a few photos of the nature. Keep this place in mind when the snow falls later this winter!