BTW, I did some Google queries and I found that the expression "Poza utartym szlakiem" seems to be quite common (54 300 results, many of those relating to a book with that title, but also used in other contexts).
"poza utartym szlakiem" - Google Search
"Z dala od utartych szlaków" also has a considerable amount of results.
Both have the same literal and metaphorical meaning as the English 'off the beaten track'.
I don't know if they can be qualified as idioms in Polish, though. Basic Google researched shows that there are different definitions of idiom. According to either of those from:
Idiom a frazeologizm - Poradnia językowa PWN
one could even argue that even English 'off the beaten track' is not an idiom, since you can infer the meaning of the expression from its parts.
On the other hand, using the definition from:
IDIOM | meaning in the Cambridge English Dictionary
you'd have to classify both as idioms.