Parks/Playgrounds that we love

Kid's Castle at Deerpath Park, Vernon Hills (the playground at the Cherokee rd entrance)
This is an amazingly unique wooden playground in the form of a castle with all kinds of cool passageways and turrets.  It is great for families with kids, especially age 5 and up.  The tire swing is also a huge hit. The downside? They are tearing it down October 2015.
http://www.yelp.com/biz/kids-castle-at-deerpath-park-vernon-hills

Deerpath Park, Vernon Hills (the playground at the Warrington rd entrance)
This park has 2 different playgrounds that the kids love, including a tire swing. There is also a covered shelter and picnic tables, as well as bathroom facilities.

Playground at Independence Grove, Libertyville
Independence Grove is a beautiful, large nature preserve with a playground, bike trail, beach, bike and boat rentals, etc.

Bowen Park, Waukegan
The plus sides of this park are that it is huge with a very cool playground, a merry-go-round (which are impossible to find), and a giant rope climbing structure.
The down sides are that it is a hike to get there in Waukegan, and the area is not one that I would feel comfortable going to on just any day. They do, however, have "events", like the LiveGreen fest and Little Lake County play date in the park that are sponsored by the park district. On those days, there are a lot of people there and I feel comfortable being amongst the crowd. These events are AWESOME and in addition to the amazing playground, the kids have gotten to do really cool crafts, nature walks, etc.
http://www.waukeganparks.org/parks/parks/bowen_playground.html

Playground at Elawa Farm, Lake Forest
Elawa Farms/Middlefork Savanna is great for nature walks (with the added bonus of getting to watch metra and freight trains go by, which my boys loved when they were little). There is also a nice playground and the Discovery Wildlife Center where you can see snakes, iguanas, and other creatures.

Mellody Farm Nature Preserve
http://lfola.org/ourlands/mellofyfarm.aspx
Great nature preserve for hikes and nature walks and fort-building.
They sponsor a "play out" event in May that has a bunch of fun nature events.

Warren Township Park ("rainbow park")
In Gurnee, located on Almond next to Warren Township High School.
A few playground structures, a rope-climbing structure, a large sand pit.