What is the best way to put the sentence in the active voice while still maintaining its meaning?

before the main performance, the comedian was entertained by the audience.

Either:

Before the main performance, the audience entertained the comedian.

Or:

The audience entertained the comedian before the main performance.

Up to you which sounds better or seems closer to the original. Active voice usually just means avoiding words that make the person doing the action not the subject (i.e. The audience doing the entertaining not the subject (the comedian))

These options retain as much of the original sentence as possible, but synonyms aren't bad if you want to use them either

Hospital, bank, museum, cinema,park,restaurant