1- shop some ottoman and turkish traditional clothes, jewels, sourvenirs etc.
2- eat lahmacun and drink ayran with it. and of course eat any kind of kebab.
3- have fun and joy at nightlife, you have thousand of options(seriosly, thousands.)
4- pass to the asian side of istanbul, and have a little trip around kadikoy.
1- keep your attention at your handbangs, wallets and other personal belongings. it was toooo risky few years ago, and there's not so much snatching now, but still keep this in your mind.
2- dont buy the thing you want from the label price, offer some bargain, trust me you'll buy it cheaper.