As nouns the difference between milestone and activity
is that
milestone is a stone milepost (or by extension in other materials), one of a series of numbered markers placed along a road at regular intervals, typically at the side of the road or in a median while
activity is the state or quality of being active; nimbleness; agility; vigorous action or operation; energy; active force; as, an increasing variety of human activities.
As a verb milestone
is to place milestones along (a road, etc).