5 nights is way too short to discover Istanbul. The capital of three former empires, Istanbul is like a piece of bachlava, many layers of richness and sweetness. The appetite grows by eating it and still, after finishing all the plate, you want some more.
I have already posted about the extraordinary shashlik kebab in Köşebaşi, the other two restaurants I would highly recommend would be Poseidon (Çevdetpasa Cad. 58, tel. 0212/263-3823) and Uskumru (55 Körfez Cd Anadoluhisarı, tel. 0216 460 1000). They are both fish restaurants that serve traditional seafood mezes and then seafood prepared in any way you want. They both face Bosphorus. Just Poseidon is located in the trendy Bebek area while Uskumru is on the Asian side of Istanbul that can be best reached by a water taxi. Both are not really hidden addresses, but most of the clientele were locals – and we all know that is always a good sign 😉