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
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
This is incredibly difficult to draw as shown by my sketchbook efforts above so I cheated and traced the shape then cut it out
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.
(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