Does anyone know of any Jewish religious works written in Spanish or Ladino by a prominent Jewish scholar (eg, Ibn Ezra, etc.)?
The Library of Congress has a large catalog of works written in Ladino, many of which were written by prominent scholars. It might take a while to sift through the database, as there is a lot of material there.
Matthias Lehmann's book on Ladino rabbinic literature might be a good place to start... He discusses the context and history of mussar literature (writings on Jewish ethical and spiritual development) in Ladino. It looks like many of the works he discusses are translations from Hebrew.
And I second the "vote" for the Me'am Lo'ez! It is my favourite commentary to learn Pirqei Avoth with. By the way, it was started by R. Culi, but he unfortunately died in the middle of his commentary on Exodus. The project was completed by other contemporary rabbis, including Yitshaq Magriso, Yitshaq Agruiti, and Rahamim Mitrani.
You could also try Menashe ben Yisra'el.
Here's one of his works in Spanish: De la Fragilidad Humana y Inclinacion del Hombre al Peccado
And, here's a web page of his works in general: Menashe ben Yisra'el Collection
This is probably not what you're looking for, but there are apparently a few words of Spanish in the Zohar.