Let us go to a restaurant and have . . . . . . . . Coffee.

In English, articles are used to indicate the specificity of a noun.

"A" and "an" are indefinite articles used before singular, countable nouns when referring to something nonspecific.

"The" is a definite article used before specific nouns that are known to both the speaker and the listener.

In this sentence, "coffee" is an uncountable noun when referring to the substance in general. Therefore, no article is needed before "coffee."

The correct sentence is "Let us go to a restaurant and have coffee."