Finally rather than at last. Clever you is fine.

The sentences you've provided need some adjustments for clarity and grammatical correctness. Here's a corrected version First, we have to decide when and where to hold the party. Who should we invite? And finally, what will we do on that day? Corrections explained: Punctuation and structure: The original sentence "First we have to decide when and where to hold the party?" is a statement, so a question mark is unnecessary. It should end with a period. In "Who to invite?", the phrasing is incomplete. A clearer structure would be "Who should we invite?" to make it a full question. In "at last what will you do on that day?", it’s better to use "And finally, what will we do on that day?" for a more natural transition and clarity. These changes improve the sentence flow and make it grammatically correct.

1)It's ok, maybe you should write it in that way: First, we have to decide where and when to hold the party, who to invite, at last, what you will do on that day. 2) Maybe 'you are such a clever boy/girl' would be better.

There should be no question marks because you haven't asked any question: "We first have to decide when and where to hold the party, then WHOM to invite, and finally what to do on that day. Yes, we often use phrases, like "clever you", that contain nothing but an adjective and a noun. Here are some more examples: "sweet thing" "beautiful day" "good luck", "good day" "pretty girl" "happy you" There are no rules. You can invent your own.