The Denville option is one I'll keep in mind since I will have over 8 hours of driving just to that point so depending on how I feel, the train ride might be perfect. I did a bit of research and found that overnight parking at the Tonnelle Ave. light rail station for $3/day may be an option as well. From what I can tell though, both options seem to be park at your own risk, and aren't really setup for long-term parking (e.g. fences, security). I don't have a stand out vehicle or anything and I know that things can happen anywhere you park, even your own driveway, so perhaps I'm overthinking this or reading too many paranoid posts.
To get closer to NYC, it appears that the Seacaucus terminal is popular, but with parking at nearly $30/day I feel I should be getting additional services included (e.g. oil change, wash and wax) LOL
I haven't looked into the airport area yet, as I assume they have tons of long-term parking, but I am contemplating taking the Port Imperial ferry, I realize it's not the most cost effective option, but I can rationalize the added expense through my entertainment budget.