The two sentences you presented use the so-called anticipatory “there” as subject, where the notional subject is the noun phrase that follows the linking verb “is” or “are.” The anticipatory “there,” as you may recall, carries little or no independent meaning and simply points forward to the notional subject that’s positioned later in the sentence to give it end-weight or emphasis.
The rule for deciding whether the notional subject is singular (thus needing “is”) or plural (thus needing “are) is very simple: it’s singular when the head noun is singular in form or notionally singular, and it’s plural when the head noun is plural in form of notionally plural.
In the first sentence in question, “There is plenty to enjoy in the cultural arena,” the head noun of the noun phrase, “plenty,” is notionally singular, so the use of the singular form linking verb “is” is correct. In the second sentence, “There are countless attractions, too, trendy and vibrant,” the nominal noun of the noun phrase, “attractions,” is plural, so the use of the plural form linking verb “are” is correct.
RELATED READING:The wisdom of routinely avoiding anticipatory “there is/are” clauses
