This is a YouTube clip from Bernstein’s 70th birthday celebration at Tanglewood, in 1988:
Jerry Hadley is the tenor; he sang the role in the Broadway productions. Seiji Ozawa is the conductor; he was a good friend of Bernstein’s.
Bernstein was a composer, conductor, and performer (pianist). You can find records of of him doing all three at once. He was also an influential educator:
Then there’s this little operetta: