General opposites:
* Inconspicuous
* Unnoticeable
* Unobtrusive
* Discrete
* Hidden
* Concealed
* Secret
* Unremarkable
* Ordinary
* Plain
More specific opposites, depending on context:
* Subtle (if referring to a style or appearance)
* Modest (if referring to behavior)
* Unassuming (if referring to personality)
* Indistinguishable (if referring to blending in)
The best antonym will depend on the specific context of how "conspicuous" is being used.