My group has a preferred tailgate location on the U of M golf course. It's a little bit of a hike to the stadium, but there is an abundance of spots where you can back up to trees or fairways and the parking attendents are generally pretty passive. Unfortunately, it looks like weather might force an alternate location Saturday.
In the past, we've used the Salvation Army lot as our back up. It's only $20 to park and you can get there as early as you want (plus they let you use their bathrooms), but the atmosphere is rather dull... so I'm looking for some alternate suggestions.