Free park and ride on nights and weekends: http://chicago.cubs.mlb.com/chc/ballpark/transportation/
Last few times I went to Wrigley I did the park and ride and it worked great, although that was a few years ago when they did it in a different location. Someone who's done it more recently can speak to how well the current setup works.
I saw that and wondered if it would be a good idea. I wonder how quickly it gets filled up. Being that it's a Sunday night game, might there be less people at the game? I swear when I looked at the time of the game when I got the tickets it said 1:20 and now when I look it says 7:08. That 5 hour drive home after the game will be fun no doubt.