If you have any extra time, Door County is a must-see. Head north of the Sturgeon Bay canal and it is hard to go wrong (~45 minutes to Sturgeon Bay, 1.5 hours to the tip of the peninsula). If it's a nice day, visit Cave Point or Peninsula State Park. On the way up to Egg Harbor & Fish Creek, be sure to stop and get some coffee at the Door County Coffee & Tea Company in Carlsville. There are cute shops and things to do in Egg Harbor & Fish Creek. Plenty of spots to swim including Clark Lake if the bay or Lake Michigan are too cold for you. Plenty of wineries and orchards (little late for cherries, might be good timing for apples). A nice little corner in the middle of the peninsula includes Stone's Throw Winery, PC Junction (restaurant where food is served by model train), and Peninsula Pub (good burgers). Shartner's and Wood Orchard markets are nice Door County farm markets with cherry things galore.
Hard to mention Green Bay without mentioning the Packers. There is the hall of fame, a restaurant/bar, tours, pro shop, and next door to the Titletown District.
Spend some time down by Lambeau Field. The Titletown District has lots to do including breweries, restaurants and driving around the stadium.
Pleasant hiking area along a creek. A good portion of the path is paved and there are additional mountain bike trails. Also connects to Triangle Hill which has a frisbee golf course in the summer.
A zoo. Also connected to the reforestation camp which has hiking, biking, a zipline, and a ropes course.
On the way up to Door County is a nice park with camping and a boat launch. Head down the steep bluffs to the bay for a great view.
A small local amusement park on the bay. $0.25 per ticket (some rides require multiple tickets) along with Elvis's favorite roller coaster, the Zippin Pippin. There is also a wildlife sanctuary nearby.
It is a garden that is botanical.
A walk along the Fox River in downtown Green Bay. Goes past plenty of restaurants, bars, and shops.