Every country with a coast needs a swish beach resort and Portorož (Portorose in Italian) is Slovenia’s. There is a sense that this could be anywhere in the northern Mediterranean, which is fine if that generic experience is what you're looking for. But Slovenia's other coastal towns have significantly more charm and character.
Portorož’s beaches are relatively clean (if wall-to-wall with people in summer), and there are spas and wellness centres where you can take the waters or cover yourself in curative mud. The vast array of accommodation options makes Portorož a useful (if less atmospheric) fall-back if everything’s full in Piran, which is only 4km up the road.