Best goodbye song ever is "We've Only Just Begun" by the Caroenters. I know it's more traditionally a wedding song, but the lyrics are superb for a retirement send off.
We've only just begun to live
White lace and promises
A kiss for luck and we're on our way
(We've only begun)
Before the risin' sun, we fly
So many roads to choose
We'll start out walkin' and learn to run
(And yes, we've just begun)
Sharing horizons that are new to us
Watching the signs along the way
Talkin' it over, just the two of us
Workin' together day to day
Together
And when the evening comes, we smile
So much of life ahead
We'll find a place where there's room to grow
(And yes, we've just begun)