There are no evening discount tickets, whether you are staying onsite or not.
As DLR said, though, if you are attending a convention locally, sometimes the convention organizers arrange to have special tickets available for their attendees, to go to the parks after 5 p.m.
And as Royal said, depending on how many days your parkhopper ticket has on it, it gets progressively cheaper to add extra days. If you were going to buy 4-day parkhoppers but you'll be there by, say, 5 PM the night before and you're debating whether it's worth it to add a day, so you can go in the parks the night you arrive... I'd say it's totally worth the $10 to add another day. You can go in, look around and explore, ride a couple things, and enjoy the fireworks if they're on that night.