The main poetic device in "How Happy is the Little Stone" is personification. Personification is a figure of speech that gives human qualities to animals, objects, or abstract things/ideas. The stone is given human qualities to illustrate how the stone, or we who observe the stone, would consider stone's purpose in the universe.
The stone is "happy" because it has a carefree existence. It rambles without care for a career or some urgent (exigent) needs. The speaker of the poem gives the stone more human qualities. It "rambles," doesn't "care," and never "fears."
The speaker uses a simile (a figure of speech using "like" or "as" to compare two different things) to compare the stone's independence to that of the sun's independence. The speaker envies the stone's independence, again treating the stone as if it were human (or conscious).
This poem is an ode. And ode is a lyric poem that praises or glorifies some aspect, quality, object, or being. This is usually something in nature. The speaker praises the stone for its simplicity and immutability. Relative to mortal things like a flower or a human being, the stone changes very little and lasts a lot longer. The speaker longs to be like the stone because the stone does not have to deal with human cares and concerns. The benefit of being like the stone is that existence would be carefree. The downside is that the life of a stone is a thoughtless existence; so, you might not be conscious enough to know you were carefree. A great poem that illustrates the aphorism "ignorance is bliss."