Don’t write “I can’t believe you gave me this” unless it’s a compliment. Don’t say you’re “too busy” to write—they know you’re lying because you’re posting on Instagram.
Don’t write a generic “Thank you for your generosity” without anything else. That’s what you say to a vending machine. You are talking to people who love you.
And please don’t mention that you already returned the gift for cash. We all do it, but we don’t say it. That’s a wedding-level secret.