You can try staying a night at the Taj hotel, lake Pichola. The hotel offers spectacular view of the lake and you won't feel anything less than a Royality
The best times to visit Istanbul are from March to May and between September and November. That's when crowds at the city's attractions are manageable, room rates are average and daytime temperatures generally sit in the 60s and 70s.
Here is everything that might help you with the trip - https://www.tripoto.com/istanbul