Mine will be layered versions of the truth, depending on who is receiving it. But if these people are close friends, then I'd give them the truth. ''You guys know we've always been a bit on the frugal side. Well, all those years of driving cheap older cars and living below our means have paid off. We've run the numbers and we believe that if we continue to be frugal and invest wisely, we have more than enough money to last us the rest of our lives. So, we are retired!"
On the other end of the spectrum will be the people I don't trust not to be assholes or greedy, or both, or with whom I just don't want a protracted and personal conversation. They will get nothing--not a lie, just not a formal announcement that we are both retiring--, and if they ask a question, will get a glib, frivolous response. "Why don't you go to work anymore?" "It seems like that would be a bit awkward after I quit!" "Wait, so you don't work?" "I definitely work. In the garden, on my writing, volunteering at the library! I don't we we can ever really escape work." That sort of thing. Most people would get something closer to the first response though.
For me, saying it was a sabbatical would feel like a lie, because that implies a clear intent to go back to work, which wouldn't be accurate. And with my friends, if I can't be open about something like this, well... Im not sure they are really friends then.