First, we should discard the easy translation of "success" because "success" is convenient but it is not a good interpretation in this context.
Second, if we look in a Spanish dictionary, we find some interesting things.
(Del lat. exĭtus, salida).
m. Resultado feliz de un negocio, actuación, etc.
m. Buena aceptación que tiene alguien o algo.
m. p. us. Fin o terminación de un negocio o asunto.
--Real Academia Española
The etymology of the word is from Latin exĭtus "an exit". The third definition is merely the ending of a negotiation, transaction, or issue: there is no positive or negative connotation. The first two definitions, however, have specific, and positive, connotations. Feliz is happy, and buena is good. Therefore, when trying to find an analogous phrase in English, focus on endings or results (definition 1. "Resultado") that are happy or good. "Success" is only one incarnation of a happy or good result, so it is much too narrow as a starting point for your search.
Third, éxitos is the plural form. That might not matter, but it might help you find an analogous phrase.
Not a great fit
Because of the above, I am not a fan of "excelsior" because it speaks to a process, not a result. I do not think "good luck" is similar to éxitos because there does not seem to be an connotation of luck (suerte) in éxito.
"Have a good day" was one suggestion. In a comment, however, you wrote that the cup "also has 'Felíz día' [written on it] and a drawn picture of a kitty cat." Felíz día literally means "Happy day", of course. So, "have a good day" would be a redundant interpretation of éxitos.
"Happy trails!" is an old expression, but I do not think it a good fit because it refers to a process, rather than a result--but it is plural, which is an advantage.
Awkward, but accurate suggestions
- All the best!
- Best Wishes
- Fare thee well!
- Go forth and prosper
I think one of the problems is that the Spanish word implies "have" éxitos, at least when translated into English. Therefore, most English phrases will lack the concision and poetry of a single word: éxitos.
Google translate suggests some interesting words:
- prosperidad ("Go forth and prosper" is not a bad suggestion)
- hit (as in a musical "hit")
I cannot think of a good analogous English word or phrase, and I think you should invent something. Some ideas:
- Be triumphant
- [May you] realize your dreams (realización)
- Have prosperity
My personal favorite right now is rooted in United States history, "win glorious triumphs," and all three words are potential interpretations of éxitos.
Far better it is to dare mighty things, to win glorious triumphs even though checkered by failure, than to rank with those timid spirits who neither enjoy nor suffer much because they live in the gray twilight that knows neither victory nor defeat.
--Theodore Roosevelt (emphasis added)