Mostly songs, including TV series theme songs and movie soundtracks. The theme from "Odd Couple" instantly reminds me of my brother who died last year.
There is also a particular aircraft engine, the Pratt & Whitney "Twin Wasp," that takes me back to my pre-teen years. Two of them powered C-47s/DC-3s, would see dozens of them a week in the late 60s. There are still a fair number of them flying today, I recognize them immediately without having to even look. In fact it happened just last week.
The smell of burning peat, the very essence of Ireland and memories of amazing times I've had there over the past thirty-five plus years.