So I'm going to try to make it out to Detroit for Opening Day again this year. I want to take my girlfriend to a cider mill, but I can't seem to find any that are open year round (most are just Labor Day-Thanksgiving). Anyone know of any?

I've never heard of any. As far as I know, apples are pretty seasonal around here.

Go to Kroger. Buy apple juice and donuts. Break into a cider mill and eat and drink there.
Close enough.

You might find a mill/store open, but not producing cider until fall. Kinda takes the fun out of it.
http://dearborn.patch.com/articles/your-guide-to-cider-mills-in-southeast-michigan

Maybe head to Greenfield Village the day before or after opening day?
Kinda pricey, but worth it, imho.

Cool

Oh yeah, definitely gonna do Greenfield Village. I've been there a bunch, but it's always fun to take someone else there for the first time. And I did find this place online called Past Tense in Lapeer that is open year round, but like you said, they're probably not producing cider in April which does kinda take the fun out of it. Damm.

LOL. I was hoping it wouldn't come down to that but, yeah.

<Go to Kroger. Buy apple juice and donuts. Break into a cider mill and eat and drink there.
Close enough.>

Classic....That made me laugh.

How about....
Go to Kroger. Buy apple juice and donuts. Then drive over to any Tractor and Feed Supply store. Jump the fence (if closed) and sit behind the wheel of a big tractor. Open the bag of donuts and pour yourself some cider.

Continue waiting there until police arrive.