A lot depends on how much your budget is and what you are going to do during the week.

Eating out for instance, there are plenty of places where a decent bowl of Ramen can be had for under a 1000 yen or multiple Michelin starred restaurants that run into hundreds of dollars for the meal.

My suggestion would be to get about 20000 Yen at an ATM on arrival at the airport. Use the ATM not the cash exchange for better rates.

Then get yourself a Suica or Pasmo card at the airport and load it up with 5000 Yen initially. The Suica or Pasmo cards are used for tapping on and off of subways, trains and buses and can also be used to pay for goods in conbini’s and other places, including some restaurants.

Top your Suica or Pasmo card up as required and get Yen out of the ATM at 7-Eleven as required. 7-Eleven are everywhere and all have ATM.