I travel to Green Bay for business quite frequently. I'm no expert, but here's what I've found. (By the way, nothing in Green Bay is more than 10-15 minutes away).
Joe's Barbeque- This is seriously some of the best BBQ I've found, anywhere. The guy's from Texas, couldn't find good 'que when he moved to GB so he started doing his own. Little take out place with only a couple of tables. Sell's it by the 1/2lb and I don't think anything is more the $5. If you like BBQ, go here for lunch.
Hinterland is fairly upscale with a very cool, trendy vibe to it. Locally sourced ingredients and definitely for 'foodies'. The menu changes frequently.
Republic is a steakhouse DT. Cool, clubby sort of atmosphere. I found them to be a little inconsistent, but I have had some excellent steaks there. Some of the seafood has been pretty good to. I almost always get a NY done "Pittsburgh style", which is nothing more than crusted in kosher salt, but it's damn good!
I've also had good meals at Chefusion. I would avoid Regatta and most of the sushi places I've had are just OK.
Finally, even though it's a chain, I do like Old Chicago. If I just want to have a few beers in front of some nice TV's with pretty good bar food, that's where I go (it's super close to my hotel).