In the sentence "Let's walk around.", is around a noun? Also, in the sentence "Let's walk around the block.", what part of speech is around?

Answer 1

Many words can be identified as adverbs AND prepositions, depending on how they're used in the sentence.

In your first sentence, "around" is an adverb. It's telling where the verb is taking place.

In the second sentence, "around" is a preposition; the entire prepositional phrase is "around the block" and it's being used adverbially.

http://grammar.ccc.commnet.edu/grammar/adverbs.htm

http://grammar.ccc.commnet.edu/grammar/prepositions.htm

Answer 2

No, "around" tells where you are walking, so it modifies the verb.

"Around the block" is a prepositional phrase telling where you are walking.... so around is a what ?