Last year we found a free thermal bath on the Danube near Hârșova in Romania. This seems to be the Bulgarian counterpart albeit a few hundred kilometers upstream.
It’s one of the sulfurous springs with the characteristic egg smell and white slimy stuff and nice blue green algae. This does not scare off the locals from enjoying a bath here. The water source is tapped with a metal pipe and comes out at about 45°C and barely cools down before it hits the first pool with an impressive flow rate. Here you can use one of the upcycled laundry detergent containers as jugs to rinse yourself off. From the first pool the water drains to the main pool or alternatively straight into the reed bushes if the pool gets too hot.
Beside two small concrete buildings next to the pool, there is a bigger construction with individual bath tubs. A spa from the distant past.
Toplata voda, Svishtov, Bulgaria: If you don't mind sulfurous hot springs, this is well worth a small detour especially if you're planing on using the ferry in Svishtov. – bert