There have been decent erotic massage studios in Zagreb offering happy endings and more a few years back. Sadly, most of them have been shut down. Zagreb’s erotic masseuses have moved to the classifieds, but some studios remained open. I’d go with one of the Thai spots like Thai Centra Thalea (https://thaicentarthalea.com/) at Bogovićeva ul. 7, or Thai Harmony (https://thai-harmony.com.hr/kontakt/) at Šubićeva ul. 21.
Chances for a good old rub and tug are slim, but the massage is good. It will cost you around €30 for an hour and it might be possible to negotiate “extras'' directly with the masseuse. Tread carefully, though: this isn’t Bangkok after all.