I'm from Turkey and I live in İstanbul for 10 years, I will gladly answer that..
Will restaurants be closed during the day?
As far as I can see, this depends on the location. In some small and conservative cities, most of the restaurants will be closed until Iftar (or some of them closed all day long). In İstanbul, most of them will be open during the day. You will never be in a situation like "I couldn't find any food to eat" no matter where you visit by the way. Also, most of bars and pubs will be closed during Ramadan even if they are located in a touristy place like Taksim Square.
Will tourist attractions be closed during the day?
If your attraction is appropriate for Ramadan, you will be fine doing it. For example; if you drink alcohol on the street (since alcohol is haram in İslam, some people are more sensitive during Ramadan about it) or you wear low-cut dress or really short miniskirt, this will not be viewed as appropriate for Ramadan (in most people's minds) and you might get reactions from some people (I know this because it happened to my girlfriend).
I checked all the attraction types in TripAdvisor and no, you will not miss any of those activities just because of Ramadan.
Will I miss out on any other opportunities that would normally be
available when it is not Ramadan?
I don't think so. All tourist activities will be the same comparing with other parts of a year.
But please be aware
Unfortunately, some people will be unnecessarily angry since they will be hungry from Suhur to Iftar and they do their daily jobs as well, their tolerance of some usual things will be decreasing during Ramadan. I strongly suggest you to not mess with this kind of people.