If antique shopping is your thing, then ‘the largest outdoor antique market in Japan’ is for you. It’s a regular event held on the first and third Sunday of each month.
The Oedo Antique Market takes place at the Tokyo International Forum which is right next to Yurakucho Station on the JR Yamanote line. Note that there will be no market in the case of rain.
There is also a monthly sister event in Yoyogi Park, though this has been put on hold until autumn 2019.