If you go to the rest stop to the west of mayohiga and then go west you should end up in the "misty area" that was blocked off before beating shinki.
http://i.imgur.com/JzYh0A1.jpg (the path going west near the middle of the map)
here is a list of all the post game battles.
Chen (at Mayohiga)
Ran (at Yukari's House use the gap map and change pages)
Yukari (at Yukari's House)
Reimu (Hakurei Shrine)
Marisa (Her house in the FoM)
Medicine (Suzuran Fields)
Cirno (In the Icy Cavern)
Faeries of Light (Behind the Hakurei Shrine)
Sakuya (SDM Kitchen)
Nitori (Outside her workshop)
Yuugi (Outside the entrance to the City in Former Hell)
Utsuho (Before the hallway leading the the Hell of Blazing Fires Control Area)
Yuuka (At her house)
Shou (At the Myouren Temple)
Sanae (At the Moriya Shrine)
Sara (at the end of the cave to Makai)
Yuki/Mai/Yumeko/Shinki (end game Rematch fights)
Letty (Bottom of the Ice Cavern)
Alice (at her house)
Seiga (In the Myouren Temple Cave as a part of the Kasen Sidequest)
Sukuna (In the Hakurei Shrine as a part of the Kasen Sidequest)
Sukuna Again (Outside the Hakurei Shrine near Genji after the Kasen Sidequest)
Also beating Yukari unlocks the ability to battle people online so that's where most of the post game content will be.