Centralia is a good one. Abandoned mining town. It's the town the series Silent Hill is based around.

Graffiti Highway. Abandoned Highway right next to Centralia. Covered in graffiti and (sadly) dirt mounds now as the government tries to get tourists away from the area.

Brynesville. Another ghost town that's actually right below Centralia. No one talks about it however, seems to be largely unknown. http://byrnesville.com/

Turnpike Tunnels. An old unused stretch of the old Pennsylvania Turnpike that you're able to walk/bike on. There's a few tunnels you can walk through also. I'm pretty sure you're also able to access the service areas in the tunnels although I never tried to due rust, moisture, and other disrepair.

These are just a few common areas that I know. If you're near philly then exploring exploring the abandoned piers along the delaware river is always fun.