I have never been to any of the cities and to be honest I wasn't entirely sure where Helsinki was. I did a little preliminary research and noted what my first "gut reaction" was to the cities.
My first thought was to take a mapping angle so that the travel guide was useful even before it was opened. I do love hand drawn maps such as those that architect Liz Steel draws (like this) but my map drawing skills are nowhere near as attractive as hers.
but I found a view of Helsinki which was reminiscent of the Londoners view of the world
(ok the original was New York & done by Steinberg who was brilliant but this is the one I saw first)
I think this could work (and wow Helsinki has its own typeface. This is just a rough it would need to be thicker to make a proper title) It would need some simple colour to make it eyecatching, probably mainly blues, greens and stone colours. Problem is that I can't find similar photos for the other cities
Istanbul has some great skylines
and then I found a fantastic mosaic
This is incredibly difficult to draw as shown by my sketchbook efforts above so I cheated and traced the shape then cut it out
This set me to explore patterns typical to the three cultures,
Helsinki has the Sibelius monument which is rather lovely and has an interesting pattern but I think I prefer the map version I did above. Milan has an interesting building for trade fairs in the FieraMilano which could give a modern pattern to overlay on pictures of older buildings conveying the mixture of experiences available to the visitor.
and a very interesting view of the Cathedral from above, there were photos of people(tourists) walking on the roof
but I dont think it works yet so I looked at other impressive buildings and found the Galleria Vittorio Emanuele
which can be used with the Bodoni font. I like this and think it would be worth developing
(the colours aren't right but I added them in case the image didn't make sense)
Which just leaves Istanbul. I drew an outline of typical buildings behind the mosaic.
This can then have a title above
I then added colour but wasn't sure how the lettering best fitted so I have 3 prototypes
I think I prefer the middle larger image