If you’re out for the day, I think you literally just need to say “I’m not in the office today, but I’ll respond to your message as soon as I’m back.” If people need an answer to something today, they’re smart enough to figure out they need to ask somebody else.
But perhaps we have it all wrong, and are simply enslaving ourselves further to technology by toiling over OOOs that are personality-packed, marketing-friendly perfection. Maybe we need to be altogether more standoffish if we want to make our OOOs really work for us? NYU Professor Meredith Broussard, who’s the author of Artificial Unintelligence: How Computers Misunderstand the World, takes the inspiration for her OOO from US writer, poet and children’s author E.B. White, who once turned down an invitation from President Eisenhower with the words “I must decline, for secret reasons”. Accordingly, Broussard’s OOO reads simply: “I am out of the office, for secret reasons.”

Start by recognizing your backup contacts for the time when you are out of the office. Make sure that, when needed, they can be available to help customers instead of you. Meeting with your co-workers and making everything clear should be one of your top priorities. The person who covers you while you are gone should not find that out by receiving an email out of nowhere. Be professional and plan everything properly — you are about to take a break, after all.
You crossed everything off your to-do list and cleared out your inbox. There’s one last thing for you to do: Write and activate the out-of-office message on your email.