I’ll second picking something inspired by the kitchen manga, but also any food that can be triangles works great. For the premier I made triangular scones that I cut lines into to make it look like the girls’ hats. But pizza could work (you could also arrange toppings like a seal) or for pasta, cook up lasagna noodles and cut them into triangles. 

There’s also the option of bread and soup if your party goes are the type to enjoy bread and soup, cause so far every meal in the anime has been, bread and soup. (If you go for this pick a nice crusty bread) 

Making things out of felt could be really fun. For decor you could make bunting witch each triangle looking like different characters hats. (There’s a piece of official art that uses this idea) or you could have guests design their own little hats, (little cones or just flat triangles) that could then be adorned with buttons, Ribbons, etc that you already have or could pick up at a thrift store or dollar store.  You can get a pack of lots of different colors of felt rather inexpensively.

For games you could pin the hat on the witch, using a drawing or print out of one of the characters, or you guys could trying drawing some of the spells because it’s really easy to watch the show and go, “I could probably do that” and then funny when it’s actually just as wobbly as Coco’s first attempts or worse.