Keep an object away from another object?

With a top down oriented game, is there a way to keep enemies away from certain environmental pieces?

Specifically, how would I go about telling an enemy that once it gets within a specific range to another object, to then travel in the opposite direction?

A few ways off the top of my head. 1. Use the distance expression 2. Create a sprite, like a circle, that goes around the object. Make it invisible and check for collision with it. Collisions still work on invisible objects.