In the walking distance of the center of Tallinn, there is a prison named Patarei Merekindlus which is well worth visiting if you have an appetite for quirky abandoned places. It has been turned into a semi-touristy (low-key) graffiti art exhibition but this shouldn’t discourage you from dropping by.
Category: Quirky
Located in eastern Latvia, an interesting sight for off the beaten path exploring is the abandoned Soviet military town Skrunda-1. I was there in 2015 and it was very non-touristy then.