"We merely skimmed the surface of the cities." Does this make sense?
Solution 1:
It is an idiomatic use that is reliably understood in the manner you suggest: that you only had time to see or feel a few aspects of the cities.
Skim = to read or look at something quickly to understand the main points, without studying it in detail:
Several dictionaries of idiom show “skim the surface” as something like:
skim the surface (of something)
To do, engage with, or understand something to only a minimal or superficial degree.
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