This is a great day trip, particularly as the ferry ride is so enjoyable. If you manage to catch an early ferry, you will be able to spend the morning on Heybeliada and the afternoon on Büyükada (or vice versa). The islands are busiest on holidays and in summer, and ferries can be unpleasantly crowded on weekends at this time; consider visiting midweek instead.
The largest island in the group, Büyükada (Great Island), is impressive viewed from the ferry, with gingerbread villas climbing up the slopes of the hill and the bulbous twin cupolas of the Splendid Otel providing an unmistakable landmark. The ferry terminal is an attractive building in the Ottoman Revival style; it dates from 1899.