Is there any difference in phrases usage? Which one is better for title of a story? The story is about tourists.
I agree with others in that "excursion over the city" would have to be literally over the city, most likely via flying.
"Excursion around the city" implies that the tourists will be exploring within the city, but it also suggests that they are already there ("Let's take a trip around the city.")
"Excursion into the city implies that the tourists are going into a city that they will then explore.
It really depends on the setting and context of these tourists as you set up your story.
One would normally take an excursion to a city, but if you were flying and never landing you could take an excursion over a city. Or you could take an excursion around a city. It's just a short trip. As NOAD says:
excursion |ikˈskər zh ən| noun 1 a short journey or trip, esp. one engaged in as a leisure activity : an excursion to Mount Etna
And a further usage note under journey:
Excursion also applies to a brief pleasure trip, usually no more than a day in length, that returns to the place where it began (: an afternoon excursion to the zoo).
I honestly can't think of an example where you would take an excursion over a city.
I would definitely choose around in this case. It implies that they went to the city in question, and travelled around it, taking in the sights.
An excursion over the city just seems absurd. I would fly over a city, take a flight over a city, take a trip over a city (maybe, it's very ambiguous), but I'm not sure I would ever take an excursion over a city. Perhaps that's just me, though.