They basically mean the same thing, but different people will give them different shades of meaning. Here are mine, but remember that this is just one (American) opinion:
- During suggests that visiting Shanghai was one of several things you did.
- Over implies (to me) a somewhat longer stay in Shanghai.
- For tells me that you spent your entire vacation there.
Now let's see what others say.
[I edit copy and have tutored college writing.]
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