Hi,
MarkK Hanau recommended the best things in Frankfurt and there are just a few remarks I would like to add:
The flea market at the Main river has progressively become a place for commercial selling of all sorts of things, most of minor quality. Yet there are occiasional bargains found, and its setting at the Main river makes it worth a visit, if you not seriously looking for genuine antiques etc.
The touristy taking pictures "must do" things for Frankfurt are those where you will barely find locals, namely the Roemer and the Dom, and MarkK Hanau hasn't mentioned it, and I guess for a good reason, "Alt Sachsenhausen". It is a shabby Pub and bar area, kind of low class and very touristy., Yet the pubs are inside one of the nicest old authentic builings you can find in Frankfurt, make up your mind for yourself, maybe you will make a few pictures....
If you are looking for an authentic Frankfurter culiary experience inside the city (Warning, Frankfurt's cuisine can be best described as "not appealing to all tastes") I would recommend "Gemaltes Haus" or "Wagner" on Schweizer Strasse (metro: Schweizer Platz). If "Eppelwoi" (the local cider) isn't your cup of tea, there are some pleasant alternatives nearby.
For real "picture taking kind of things", I would recommend a day-trip to Heidelberg, if you haven't been there. A bit overrated and overdeveloped, yet the best "romantic" German city within day trip reach from Frankfurt. Another nice option for a Sunday is a day trip to Wiesbaden, visit the City with its Kurpark and gamble in the nicest casino I have seen (not too many, though). Even if you do not gamble, the hall and the Builing and the adjected Kurpark is a very nice place to spend a sunny sunday afternoon.
Enjoy