Your aunt promises to give you $5,000 for being such a good nephew

Your aunt promises to give you $5,000 for being such a good nephew. You agree to her offer ("Yes aunt Jennie, I will take your $5,000), but you don't promise anything in return other than go to your mailbox and get the check out of the mail. If your aunt later changes her mind and states, "I am no longer giving you the money, but I am giving my niece that money," could she legally perform this, or would you be able to successfully sue for breach of contract?  
Would the facts be any different if after being promised the $5,000, you made vacation plans (flight and hotel reservations), and you had essentially committed all of that $5,000 to your vacation plans, and that you would be out of pocket $5,000 if you couldn't get that money from your Aunt?  
[For this questions, forget about the facts in b)] What if, at the time of the initial promise, your aunt wants you to do something in return for that $5,000? She demands that you make her and her 3 girlfriends a simple lunch, which you will join them in eating. You agree (wow, $5,000 for a making and eating lunch for 4 people!). Would your aunt be able to change her mind, and break the deal in this situation, or could you successfully sue for breach of contract?