The back cover illustration of volume 17 of the manga is a shout out to the promotional poster for Urusei Yatsura 2: Beautiful Dreamer, with Excel, Miwa Shiouji, and Umi Rengaya in place of Lum, Mujaki, and Baku (respectively) from the original. Also, in volume 12, "Manager" cosplays as Lum (although she claims to be wearing that tiger-striped outfit because she's a fan of the Hanshin Tigers).
In the English translation by VIZ Media, the sixth chapter of volume 20 was titled "The Waste Land", and had references to that poem in Excel's dialogue. (The original title for that chapter, however, was Kōri no Sekai (World of Ice), according to That Other Wiki.)
In the fourth chapter of volume 22, Ropponmatsu I now obeys Excel's orders, because she defeated it in combat in the previous chapter. This leads to the following dialogue:
In volume 23, in the scene where Elgala discovers that "President Excel", a.k.a. Ropponmatsu I, is a robot, she's wearing the costume of the character Mexiah from the Tenchi Muyo! spinoff Isekai No Seikishi Monogatari; the costume came from Umi's closet.
Episode 1: The first scene has a shoutout to/parody of Minky Momo (specifically the episode where the heroine was hit by a truck and died).
Episode 2: In the English dub, Watanabe poses a question using the word "surely". Iwata's response? "Don't call me Shirley; my name's Iwata."