Vanuatu is doing it differently and taking it all under water. This unique gig was started in 2003 and since then, they have had over 100,000 visitors donning scuba gears and snorkle masks hand carrying their waterproof mails and postcards under water to a sunken "post office" with a scuba-geared post man awaits to give service!
The hours, however depend largely on the weather, naturally. But in the tropics that Vanuatu enjoys year-round, you can almost be sure you can find this post office operational every day! More so on days when vacation season peaks. Check their website for more info! They also have waterproof postcards and stamps available!
If you're ever in Vanuatu and got to try this out, let us know!