Both Piran and Portorož have plenty of waterborne activities to enjoy, from swimming and diving to sailing; the latter is also possible from Izola. Portorož is also well known for its spa centres.
You can sample a portion (or all) of the Parenzana Trail, a 130km walking and cycling trail (numbered D8) that runs along the old narrow-gauge Parenzana railway, which once connected Trieste in Italy with Poreč in Croatia. About 30km of the trail runs through Slovenian Istria. See www.parenzana.info for more details, including two-, three- and five-day itinerary suggestions.