Her last opera performance will be in Richard Strauss's Der Rosenkavalier in Cologne next April, singing a role, appropriately enough, whose famous aria (the Marschallin's Monologue) sees her musing wistfully on the passing of time. Renee Fleming sets the scene for that famous monologue in this clip in one of a series of great clips on the opera at the New York Times site. And The Met's site has this reflection on the aria:
The Marschallin alone among the characters sees the future passing through the present into the past, and wonders what it means. Philosophers may say that time is only the measure of change. Poets may say "Carpe diem" -- "Grasp time while you can." But the Marschallin finds that in fact in a human life one cannot measure or grasp or hold. Each irreversible moment is already gone in the instant of becoming.
For the accountants reading this, that's like saying you can either spend your savings or have them - they're gone in the instant of enjoying them. :-)
Anyway, here's a sample from her singing the role at Covent Garden in 1986 in which she laments her quickly vanishing youth:
And this is her singing the beatiful piece from the 'Songs of the Auvergne' that you might have enjoyed in the film In My Father's Den (sadly only available at YouTube in wide screen, it seems):