When you said you met Kasen, did you mean you did her errand, or just talked to her once? Because now there are a handful of items that need to be found for her in unique places. There's a guide on this forum somewhere that will tell you where to find them.
Other than that, there are a few post-game stuff to be done. You can go around and re-battle everyone, for instance. They have stronger puppets than before you face Makai. Also, some other characters will battle you now, like Nitori and Alice. As a bonus, you can battle everyone an extra time after you beat Makai again, for as many times as you want to.
Also, the Ice Cave in the middle of Misty Lake is fully accessible now, where you can collect unique items for Patchouli's Reincarnation Table. Also, you can fight Cirno and Letty there too.
Finally, you can use the Gap Map to go to Yukari's place. This is where you can do Net Battles and such after you beat her and Ran.
I hope that this gives you plenty more to do in the game.
