Bosporus Tour
Best tour ships are moving from Ortaköy seaside (European Side). You can see all Istanbul from seeJ Its really great while you are passing under the bridge. Do not forget to take your camera:)
Location: Ortaköy (Also other seasides)
Closed: All Days Open
Cycling Tour in Prince Islands
The most beauiful island is Büyükada in prince islands (In Asian Side). Rent a bike, and tour around the island. You will live many great views in the tour. If you are couple, try double bikes for the romantic tour:)
Do not forget to take some spring water with you:) Also you can take some bread or bagel with you. While you are in ferry, there will be seagulls flying parallel to you waiting for you to throw :)
Location: Adalar (Pince Islands)
Closed: All Days Open
Shopping in Bagdat Street
It is longest and most elit shopping street of Asian Side (Just like Istiklal Street). Walking up and down is really enjoyable. You can see all kind of shops here. Also there is a seaside in the parallel street, do not return unless you walk there too.
Location: Kadıköy
Closed: All Days Open
Sapphire View Terrace
You can see Istanbul in 236 meter height. It is in the roof of Sapphire Shopping Mall, which is heighest building of Turkey. And you can try Copper simulation up there:)
Location: 4. Levent
Closed: All Days Open