Es un espectáculo para los ojos

[ROBINDESBOIS]

is there an expression similar to this one in ENGLISH?

[Rusty]

It's a feast for the eyes.
It's a treat for the eyes.
It's a sight for the eyes. (this word is used especially in the idiom 'a sight for sore eyes')

It's eye candy. (could be considered a derogatory remark, but otherwise means stunning or attractive)

[ROBINDESBOIS]

Is that also true for a natural monument?

[Rusty]

We could say those things, but it'd also be natural to say:

The monument is stunning to look at.
The monument is a beauty|a wonder|a (beautiful|wonderful) sight to behold.