There is a number of sights which could be called Main Sights of Yalta: Livadia Palace, Swallow's Nest Castle, Vorontsov Palace (also know as Alupka Palace or Vorontsov Palace in Alupka), Massandra Palace.
Other less important, but still highly recommended for a short visit sights of Yalta:
St. Mikhail Church in Oreanda (beautiful church built in the very picturesque place near Oreanda, Yalta.
Anton Chekhov House in Yalta. Russian writer Anton Chekhov lived in Yalta 5 last years of his life.
Shore excursions with a private guide will probably include all of the mentioned above sights. The time to cover it, plus of course lunch to try delicious Ukrainian cuisine and amazing Crimean wine would be about 7 hours.
See here: Yalta shore excursions with a private guide
Crimea with a private guide Sergey Sorokin: one-day and many-day tours sightseeing & hiking
Pictures © Copyright Sergey Sorokin