You should definitely see PIRAN and from there you can have a walking trip by the coast to other places if you feel like it. PORTORO® is more touristic but there is more night life and there is a camp in near town LUCIJA. You can also stay at Autocamp Belvedere in IZOLA (10% discount with European Youth Card). In SEČOVLJE there is a place where salt is produced from the sea.
More information about Slovenia: www.slovenia.info