Jimmy and Justin returned to Camp Winnipesaukee last night, singing "What's Up" by 4 Non Blondes, and Gloria Estefan's "Rhythm Is Gonna Get You" -- since it's 1993 for them.

Keegan-Michael Key stole the show, breaking up the kids's fun as a new version of their irritated camp counselor.

Jimmy: "Hey, did Mr. Fletcher look different to you?"
Justin: "Yeah, way more theatrical."

That cracked them both up. Stay to at least the 7:45 mark for another celeb cameo, with Billy Crystal playing a 12-year-old camper who joins them for another song.

Watch the full 10-minute video. Yeah, it's long, and Crystal even makes a great joke about that:

Check out the past Camp videos, and look forward to what 2018 brings these "best friends forever."

In addition to his camp cameo, Billy Crystal was a guest on the show last night, to talk Trump and share some advice for Fallon.

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