David Letterman's never looked so happy to NOT be hosting the 'Late Show,' and told us ... more power to Stephen Colbert for killing it in the ratings.

We got Dave and his massive beard out in NYC Monday -- he joked it's the first time he's been outside in a month, but by the looks of him it could be true. He let us in on his new daily grooming routine, and talked about his old late night stomping grounds.

You'd think he's missing it these days with all the Trump-related material that seems prime for a Top 10 list. Instead, he paid Colbert a nice compliment and made it clear he's thrilled to be doing nothing.

This is retirement done right.