Emirgan Park spreads over a hill above the Bosphorus, with ponds, paths, old imperial pavilions, and broad views over the water. It is especially associated with Istanbul's tulips, and remains one of the best places in the city to see spring plantings.
The park is large enough for a proper wander rather than a quick photo stop. Cafes in former hunting pavilions make it an easy break during a Bosphorus day, especially if you want greenery without leaving the city.