So... how's everyone else's session prep going?

Think of a pun like "mathemagician" or "orcward"
Build a joke NPC around the pun
Shoe-horn the NPC into whatever is going on
Think of some quest vaguely related to what they might like
That's the main quest now
- God you just described my prep in a nutshell. This is how they ended up fighting an orchestra

One of my players offered themself up as a hostage to get the others out of a losing fight alive, which was a great twist in the story but now I need to run two branches until the others can attempt a rescue
- the bad guys huddle a few feet away from the bound adventurer.
  "What the fuck are we going to do with this asshole?"
  "Uh... torture them and murder them?"
  "Bob... what the fuck is wrong with you?"
  "Well we're not feeding them. We don't have enough rations for each of us."
  "Let them go?"
  "Let them go, follow them, then ambush the party and take their food. Or eat them."
  "Well, I guess that's better than your first plan Bob."
  
  Fortunately we are playing Lancer, so it's entirely in keeping with the setting for the others to drop from orbit in big mechs to explode things until they get their friend back!
  In the meantime there was going to be a thing about working with some rebels behind enemy lines anyway, so they're going to break the hostage out of captivity at the cost of not having the resources to help in a later conflict

I usually just kept a list of what the various factions are up to. If the players were like "ok let's go see if we can convince Priscilla to smuggle the uranium with her drug stuff" I have a rough idea of what she's up to and if she'd help, or help at what cost.

I see you're new to DMing, if you think the main quest is more likely to happen :)
- Think of that bell curve meme, I think they're actually very experienced.
  Left side - thinks the main quest will happen, preps that
  Middle - thinks side quests will happen instead of main quest, preps that
  Right side - thinks about side quests but preps nothing, knowing that neither the main quest or side quests will happen, because the players latch onto a completely innocuous detail that you ad-libbed during a description
  
  I rolled an 18 in perception for the mustiness of the room.
- Main quest? Weird tangent? They're the same picture!

I gave up prepping "main quest" stuff when my players wanted to find out more about some curse affecting them, and decided the most logical thing to do is kidnap the head of their religion (who one of the players directly reports to), interrogate him, and hold him hostage. Instead of, you know, asking 😅
They eventually backed off of that plan due to concerns of some other PCs, but it was pretty close
- I want to play again with a group that comes up with reasonable plans that play to their competencies.
  I think a lot about how in a modern day magic game, the players wanted to contact another (NPC) group. They learned the NPCs were like double warded against magic, but spent a lot of time trying to punch through the wards to teleport to them. After two expensive, failed, attempts I was like "do you want a clue?". They were like yes. I was like "if you just want to talk to them, why don't you try calling them on the phone?"
- I want to play again with a group that comes up with reasonable plans that play to their competencies.
  I think a lot about how in a modern day magic game, the players wanted to contact another (NPC) group. They learned the NPCs were like double warded against magic, but spent a lot of time trying to punch through the wards to teleport to them. After two expensive, failed, attempts I was like "do you want a clue?". They were like yes. I was like "if you just want to talk to them, why don't you try calling them on the phone?"

Dang, why you go for my jugular?

I dunno, I kinda like this idea that the players will be so responsible and active over their own entertainment that they'll pick something to actively do to make something happen!
I'm still new to GMing, and one thing I encounter a lot is my local family/friends as players can be very passive in the "we gather to be entertained (everyone looks at GM like "now what?") " kinda way.
And I'm always freaking out because I can't offer some incredible satisfying plot like they'd get watching TV shows or whatever. I'm afraid of everything being not good enough lol. My previous coping mechanism was sticking closely to the book...which in this case was 50 Fathoms, a seafaring sandbox fantasy setting.
...I had to "make things happen to them" , but was REALLY afraid of somehow breaking the world (Morrowind-style lol) if I threw something too "Act 2" at them too fast.
And their captain seemed content just rolling random encounters while trading goods between ports. LOL
I think I really need to trust serendipity and stop planning so much. One of my best games was introducing my wife and her brother to Savage Worlds by just walking them into a bar and being goofy.
They made their characters after League of Legends characters. The bartender was a monkey. They caused some kind of trouble and the monkey threw a flaming ha-pooo-ken at them...oh no, the dice kept exploding! Roll for wound location..."unmentionables." Oof. They were laughing so hard.
- I kinda like this idea that the players will be so responsible and active over their own entertainment that they’ll pick something to actively do to make something happen
  This is the dream.
  Sometimes I get players that have ideas, but then they're like "oh that sounds too dangerous, nevermind", and I'm like "it's not going to be much of a game if we don't take any risks"
  It's probably partly my fault for making the dangers clear to the players. I wanted them to have an understanding of the risks and factors!
  Like one time, the players were told another faction would only help them with their problem if they dealt with a vampire that was in the local cemetery, and his little cult. This was a game of Mage, where even starting tier characters have a lot of strong options. One of the players just was like "you're asking us to punch Cthulhu in the face! I don't understand what you want us to do!"
  I was like .. there are so many options. Your character can literally control flame, a major weakness of vampires. You also have a strong alliance with a paramilitary group. You can go during the day. You have 3 other party members. One of them can open portals. Like, to places where it's daytime. Trust me, you can win a 4 v 1 fight. Maybe deal with his cult first if you don't want civilian casualties. Or maybe talk to him and see if you can negotiate.
  But she just wanted to spin her wheels and complain. Worst player I've ever had, honestly.

That's exactly where I'm at too. The side plots are more self contained and feel easier to plan since they don't have to tie in

If you're running a sandbox, you just need to review a bit ahead of the party in the various dungeons.