Ordinary, simple, vanilla
Possibly "unadorned"
"Similar" is a word that means nearly the same as "like."
Precipitation
Certify.
boundary
Height.
Plain, flatland, tundra...
A word that means the same or nearly the same as another word, such as bucket and pail.
Another word that means the same is "synonym." A synonym is a word that has the same or nearly the same meaning as another word. For example, "happy" and "joyful" are synonyms.
dealing with managing
Enlist.
Unattractive, knit, prairie, unadorned are synonyms for the various meanings of "plain".
I believe you are looking for the word synonym.