Budapest, then. You can ride the old M1 Metro line to the geothermal spas, which feels like it has never been modernised. Liberty Bridge (all the bridges in fact) in Budapest are spectacular, you can visit the ruin bars, see the catacomb/dungeons on top of Gellert Hill, bathe in the Gellert indoor spa, visit the pinball museum, go to the Budapest Jazz club, visit Margaret Island (Margit Sziget) and say hello to Suzy the pony for me.
If you are so inclined, there is also an abandoned train depot that you can break into and wander around illegally at your own risk. https://www.atlasobscura.com/places/istvantelek-train-yard
If you are swimming, (not bathing) then swimming caps are required. Flip flops are required (for safety, believe it or not) in the bathing areas.
I’ve been there three times, the last time I was there for a whole month. Prague, I did in 2 days, it was nice and I would go back, but felt like I did it in a weekend.
Just don’t clink your glasses with anyone when you drink, it’s an insult.
Budapest does and will have tourists, but it’s a substantially bigger city than Prague (at least the centre is) so it feels less crowded.
I’ll save you some time, don’t bother with the “People’s park” in Budapest, Nepliget, it’s just a bit bleak.
Oh, and “Ferfi” is Men’s, regarding mens changing rooms and toilets. Learnt that the hard way.