From a firend who lives there

some obvious, others maybe a locals only thing
but anyway a few ideas

Take the Lambeau Field tour
Visit the Packers Hall of Fame
Walk around Titletown District
Have a butter burger at Kroll's West
Have a drink at Taverne in the Sky
Visit Old City Stadium and the Packers Heritage Trail Plaza (these 2 are on the east side of town)

Take a day to explore Door County
Check out the NEW Zoo
National Railroad Museum
Heritage Hill

Restaurants we like
Stadium View
Hagemeister's
Nicky's Lionhead Restaurant (in De Pere, 3 or 4 miles south of G
St. Brendan's Inn