I just did the Keys to the Kingdom MK backstage tour and it's pretty extensive. It's ~5 hours and includes lunch (ours was at Pecos Bill's). You go into the Utilidors at the end and we got to go into HM from a side entrance and a special Jungle Cruise ride where the tour guide did all the talking about the attraction instead of the usual spiel from the skipper. I thought it was totally worth it. Behind the Seeds is also a really good value and an interesting tour.
In 2019, I did the "Undiscovered Future World" in Epcot and got to go into the former corporate suite in Spaceship Earth and it was a really fun tour, but they aren't doing the Epcot tours anymore...
I was planning to do Undiscovered World Showcase the next time I went but they discontinued it (during the pandemic, I think?) and it hasn't come back... (Yet? *Fingers Crossed*)