Always use a comma to separate a direct address (what you’re calling whomever you’re talking to) from the rest of the sentence.
The subject nouns/pronouns have been left out of the second (compound) sentence, but it is still a compound sentence. (In the first half, the subject is I; in the second half, which is an imperative sentence even though it’s a “nice” one, the subject is you.)
Hyphenate stress-free because those words function together as a single adjective to describe day.
Add a comma after day and capitalize It’s.
Good morning, everyone! Hope everyone has a positive and stress-free day, and don’t forget to smile. It’s almost the weekend!
Some people are too hyperactively cheerful, not to mention aggressive about telling everyone around them to smile. Telling other people to smile is not nearly as friendly as they seem to think it is.