Trsteno is 30 km far from Dubrovnik.  It is known for “Arboretum Trsteno”, a magnificent botanic garden, started 500 years ago by family Gučetić, is planted in wonderful Mediterranean surrounding.  Multiple daily guided trips are organized from Dubrovnik, but they are very short and I would recommend to come on your own and spend a half a day in solitude and tranquility.   
Make sure that you have enough films (or storage media).  See more here.