No visa, just an entry stamp at the border. Free for French people, 10 euros for Belgians. Belgians have to first buy a stamp from one place and then go to the customs ... all of that at 3 o'clok in the morning :o)
No particular problem.
Outdoor shops in Istanbul:
Adrenalin, Besiktas, very friendly, that's where we bought our new tent.
Kutup Ayisi, Beyoglu. Very friendly too!
Everest Outdoor, Karakoy
Cycle shops in Istanbul:
Pedal Sportif, Mimar Kemalettin Cad. No. 29, Sirkeci / İstanbul
Bosphorus cruise: Ferries timetables. You can take the ferry from Eminonu or Besiktas. There are 4 departures each day. To avoid being stuck in a touristy fishing village for 3 hours (as we did), we recommend that you take the 2nd boat (leaving at about 13:00), stop at R. Kavagi (rather than Anadolu Kavagi, the last stop). You have 1h1/2 to have lunch and visit before the first boat comes back. Being stuck 3 hours in Anadolu amongst all these restaurants for tourists wasn't very nice!