Where exactly in northern NJ are you going to be?
As someone pointed out, you could park near the GWB and utilize the buses there, however the buses do stop running around midnight on most nights leaving you to take a cab (probably "gypsy" cab if its late, which would run you about 20 bucks to cross the bridge). Good luck parking anywhere in Fort Lee that is relatively close to the GWB though.
Hoboken (or Jersey City) can be a good option due to the 24 hour service on the PATH. Again, parking is your enemy. I haven't been to Hoboken much since I lived there about 5 years back, but I'd be willing to wager that the parking situation near the PATH station is as bad, if not worse, than it always was. There are some pay lots/garages in the area though so YMMV. Jersey City can be a decent place to park and if you know the area, its pretty safe. I'd look for something around the Grove Street PATH (simply because I used to live in that area, I know that parking is not so bad on weekends, and its not a dangerous area at all) if I was thinking about going that route.
I'm not positive, but I don't think you're going to find very many (if any) trains running late on the weekends. You may find a few buses running, but I am always stuck driving or cabbing it if its going to be a late night. My friend insists that there are late buses to Rutherford, though, so this may very well depend on where in NJ you are staying.
If you're going to end up paying to park just to take the PATH, my suggestion is to drive into the city and just pay for parking there (if you can't find street parking) that way you avoid the issue. Seems like you're going to have to drive at the end of the night anyway so I have to assume that alcohol consumption is not an issue.
Anyway, if you provide more info I might be able to help more, otherwise good luck.
