Welcome to the site Maya! Sorry for not posting sooner, but it's been a busy week.
One way to find cool abandoned towns and places is to download my free Map of the Pines. It works on Mac, Windows and Linux computers, Garmin and Magellan GPS units and over two dozen smartphone apps for iOS and Android.
https://boydsmaps.com/boyds-map-of-the-pines-hd/
I spent quite awhile scouring old topo maps and aerials to find historical structures (which are shown in purple on the map). You might also enjoy the Cook topo map from the late 19th century for a historical perspective:
https://boydsmaps.com/cooks-map-of-the-pines-hd/
You'll need to install some free, open source software to use these maps - there's more info here plus a detailed tutorial:
https://boydsmaps.com/mobile-atlas-creator/
If this all seems too complicated, I will soon launch a new site for easy access to all my maps in a web browser with nothing to download or install. But you'll need to wait another month for that.
Now, for a specific suggestion of cool, abandoned ruins to visit, check out the Atlantic County Park at Estell Manor:
https://www.atlantic-county.org/parks/estell-manor-park.asp
Now, I know you said you weren't interested in parks, but this one is different. The Southern part is tourist-oriented with picnic areas and so forth. But the Northern part consists of dense woods and swamps with trails that were originally the roads and railroad tracks of a huge 100-year old munitions plant. You will find some nice ruins along the trails. But you need to venture into the dense thickets to find the really cool stuff. Most people (including the locals) have no idea of the hidden gems in there. It's like going back in time - but not easy to find. Sorry to say, you'll probably get all scratched up from the greenbriar vines, holly and laurel that has overtaken the site, so it's not for the faint of heart. And we are almost at the point where it will become inaccessible for the season (Winter is the best time to go exploring there). Just stick to the trails if this sounds too challenging - there's plenty to see there and it's never crowded.
Anyway, it is definitely worth a trip. Perhaps an hour's drive from Burlington County. Visit on a weekday and you will have the whole park to yourself. The tourists and picnicers come out on the weekends, but even then, they stay in the developed Sourthern part of the park. Enter the Northern part here on Artesian Well Road, there are plenty of places to park.
https://maps.njpinebarrens.com/#lat=39.412672513611284&lng=-74.7423565766419&z=19&type=nj2012&gpx=
Some interesting old photos of the munitions plant here:
https://www.atlantic-county.org/parks/blc-photo-collection.asp
There's a trail map here, but my map will give much more detail:
https://www.atlantic-county.org/parks/estell-manor-parks-trails.asp
But be aware - the ticks are out in large numbers already, so you need to take precautions. Are you familiar with Permthrin? You need to prepare in advance and treat your clothes (pants, shoes and socks). Do NOT spray your clothes while you're wearing them and don't spray your skin. Follow all the instructions on the can. You can find it at REI and Dick's Sporting Goods. Sometimes WalMart has it too, but you won't find it at the grocery store or drug store. After you treat your clothes, the permethrin will still be active for a few weeks, and should even survive washing (they claim 6 weeks and 6 washings, but that sounds a bit too optimistic to me). Wherever you go in the Pines, you NEED permethrin.
https://sawyer.com/products/permethrin-insect-repellent-treatment/
Have fun exploring!
