Oh, Leo!! Or anyone else interested in rock 'n roll for that matter...
I watched some of the huge Glastonbury festival in the UK (a PITA to get online here). There are 4 main stages with many well-known performers - Pyramid, Other Stage, Park, and West Holts (and umpteen other stages with bands I've mostly never heard of).
There are always surprise performers. This year the Foo Fighters showed up as The Churn Ups (which was apparently not that much of a surprise by the time they took the stage).
But what I wanted Leo to know was that Rick Astley did a surprise hour or so of all songs by - The Smiths!! (He also did another scheduled set of his own music.) There's at least one song on BBC's YouTube channel.
And Johnny Marr played with The Pretenders (also a clip on YouTube I'll get back on and post later).
The crowd goes nuts for well, just about everything! but when they saw Chrissie Hynde practically drag Paul McCartney out on stage...
I don't think he performed with any of the bands but was seen backstage a number of times. He headlined last year and had Bruce Springsteen as his surprise guest. (MTV Live showed parts of the festival during the pandemic which is where I first saw it, but I don't think they do anymore.)
And Elton John, was his show fantastic to watch live!! with a beautiful sunset in the background. I read there were 100,000+ people to see him perform, and instead of the usual blankets, lawn chairs, etc. it was SRO. He had a couple of guests that to me were unknown, as well as the lead singer of The Killers (who seemed to just be having so much fun singing with him).
The BBC televised it live and they have clips of performances on their YouTube channel.