[Sangetsu]

You'll need to fill out a form specifying your length of stay in Japan, the amount of funds you have (it better be sufficient to support yourself for your intended length of stay), and usually a return ticket. In 3 of the times I entered Japan, I was required to show the return ticket in order to get a 90 tourist visa, the 4th time I was entering with a one-year visa obtained at the Japanese Embassy, so a return ticket wasn't required.

There is probably better than a 50/50 chance that you will be denied a tourist visa, the Korea weekend strategy is well known to Japanese customs and immigration. If you pull it of, be sure to share with us how you did it. You might try buying a second set of tickets while in Korea, with your return ticket bringing you back to Korea, that's the only logical method I can think of which might work. You'll still get the 90 days, regardless of the travel date on the ticket.