I believe this topic might have been touched before but I thought I saw some erroneous information regarding taking the L to Evanston.
Here is the link to Northwestern's football website and what they say about parking:
Here is the link to the CTA and taking the Purple Line up to Evanston. It appears that there will be plenty of options to leave Ryan Field and make it back to downtown (or any stop in between) after the game:
EDIT: At the time the game will probably end the Purple line only runs down to Howard where you will have to change trains to get to another location (in my case the Loop).