A sign behind Maria says Maria+Addiction, which could be a parody of Maria Holic.
There is a sign that says Sailor Moo-, which is obviously Sailor Moon.
There are a series of percussion notes at the end of Chiri Kitsu's phrases and sentences. This is almost like a homage to the old "Phonetic Punctuation" bit by the old comedian/musician Victor Borge.
When the camera pans across guests for the Halloween party, most if not of them all are cosplaying different anime characters like yuno, hiro, sae, and miyako from Hidamari sketch, asuna from Negima!, Kagami from Lucky star, Usagi Tsukino from sailor moon, hei, and yin from darker than black, Goku and vegeta from DBZ and many others.