Conveying "under the radar" in a less colloquial way

How about best-kept secret?

The Free Dictionary:

best-kept secret: A certain aspect, fact, location, or activity, usually touristic or commercial in nature, that is or purports to be not well known to the public but deserving of praise or attention. (1) The newspaper called the restaurant the city's best-kept secret. (2) While everyone wants to visit the Ring of Kerry, the Dingle Peninsula is really one of Ireland's best-kept secrets.

One could also use well-kept secret. Thanks to @ktm5124 for the suggestion. I've heard both best-kept secret and well-kept secret. They both speak to your "under the radar".

Edit: Another possibility is off the beaten track (or path}.

The Free Dictionary:

off the beaten track (or path): not known or popular with many people; away from the frequently traveled routes. We found a nice little Italian restaurant off the beaten track.

Mundane, perhaps - ordinary, commonplace, prosaic, dull, humdrum, lacking interest or excitement.

You could say that a certain view of the city is a hidden gem. I'm actually having a hard time finding a definition of this phrase, but it's the same thing as a well-kept secret.

Travelers are often interested in the hidden gems of a city. (Restaurants, views, rooftops, etc.)