Great ideas so far. I’ve definitely done the biker movie one (as a '60s groovy theme), and the secret agent ones as well (special treats: the watch drop(s) and the Prince jokes - man, that guy really looks like him!).
I’ll watch Prince Of Space/Invasion of the Neptune Men together. And of course there’s Sandy Frank’s (he’s still alive at 92!) non-Kaiju series: Time Of The Apes,Mighty Jack,Fugitive Alien, and Starforce: Fugitive Alien II.
But for a less-obvious marathon, how about all movies featuring hypnotists (most inspired by the Bridey Murphy story and most in Season 8):
Operation Kid Brother The Undead The She-Creature I Was A Teenage Werewolf Devil Doll
Invasion U.S.A.,The Thing That Couldn’t Die, and The Pumaman can be rotated in per personal preference. *Edited to add: Oh, and Carnival Magic. Apparently Gus forgets episodes sometimes.
The Loves Of Hercules too? Probably a personal preference. You know how it is being “The Herc” and all.
I would add Devil Doll, especially since it has Bryant Haliday in it before he was in Projected Man.
By the way, Wikipedia says Haliday was the co-founder of Janus Films, which was the premier American distributer of prestige foreign films for decades! They brought Kurosawa, Truffaut, Ozu, Bergman, Fellini, Antonioni, and more to the US - their first big theatrical hit here was The Seventh Seal. Haliday only acted as a hobby - he primarily produced plays, worked at Janus, and spent the last decades of his life making TV in France. He only has 6 film credits! He’s not even British, he’s American - he’s faking his accent in his two MST films! Every day I learn something new.
Although the Russo-Finnish quadrilogy represents my favorite collection of episodes on MST3K, I really love anything having to do with musical crime capers.
With that in mind, you could engineer a neat mini-marathon with any of the following episodes:
-Untamed Youth
-Catalina Caper
-Daddy-O
-The Beatniks
-I Accuse My Parents
-Girls Town
Oh, I did forget a Herc film. I haven’t watched Season 11 enough yet to remember the episodes in it. And The Loves of Hercules is definitely a good addition.
I often like to throw in a viewing of Ed Wood with these flicks (I know it’s not MST3K, but the vibe works, and it pads out the marathon. I need a copy of the Mads’ Glen or Glenda to really round it out!)
I could pair up Mighty Jack and Alien From L.A. for no other reasons but A) They’re a hoot, and B) They both have that same green squeezebox in a funny sketch.
More power to those who’d watch everything by Francis in a row. I’m afraid that might break me. It’s enough just to get through one of them per month.
While not just MST3K, I do love watching all the holiday riffed features and shorts around the holidays in a marathon.
Another one that would be totally easy to do is an all-Paul marathon-- every riffed movie where the main character is named “Paul.” I don’t recall the specifics, but there were a LOT. Werewolf springs immediately to mind.
The MST3K Watch Together group used to do theme weekends or days-- i.e. superheroes, animation, dogs and cats, etc. That was often fun. It was easier with Rifftrax, CT, Mads, etc. included.
The key is to spread the episodes through the course of a weekend. A Coleman Francis Weekend! Example: Friday night - The Skydivers, Saturday night - Red Zone Cuba, and Sunday Afternoon - Beast of Yucca Flats (MST3K episode order)
301: Cave Dwellers
703: Deathstalker and the Warriors from Hell
1110: Wizards of the Lost Kingdom
1111: Wizards of the Lost Kingdom 2
1206: Ator, The Fighting Eagle
The “Not Even the Most Famous Person at the Family Reunion” Marathon
It Conquered the World
Operation Double 007
Beginning of the End
Village of the Giants
Parts: The Clonus Horror
Werewolf
Soultaker
Wizards of the Lost Kingdom II
Killer Fish
