My Delvers of Daggerford have returned, and after a few tweaks to get them in line with the most recent iteration of the Hack'D & Slash'D rules, and some name changes just to do it so I can make them part of the 'official' game (if such a thing exists), I sent them to a different dungeon, dropping them into the second level of the Vault of the D'Ro (which I haven't mapped yet, but I might start today). They encountered an air elemental, summoned from a censer that is permanently and magically affixed to a pedestal. The censer will summon an air elemental once per day if a statue in the same room is manipulated without speaking a command word. Since the heroes don't have the command word, and they need to pull the arms of the statue to open the doors in the room, there is no chance that they get by without summoning and fighting an air elemental. They messed with the statue, got punched by an air elemental a few times, survived to tell about it, and moved on...
But tomorrow, they may come back, wanting to investigate the next hallway. Or, they may leave and come back later, and need to open the door again - which would again summon the elemental. I mean, they could use something to wedge the door open (stakes and a mallet exist for a reason), but there's always going to be the risk of having to fight another elemental... and another... and another...
A Google search revealed (pretty quickly) that I am not the first person who has thought of this, when I remembered that my Delvers carry a bottomless satchel with them. They could always empty the satchel, use it to cover over the censer, mess with the statue, and have instant elemental in a bag. There are Reddit threads discussing this. It's not a new thing. However, I still thought of it independently, so I'm claiming credit for an old idea. Anyhow. I love this idea as a player. Now, the next time I come across a particularly fearsome monster, I open the bag, toss it into the room, and close the door. Elemental attacks. Wait one minute, and go in to fight the injured winner of the fight.
Right away, I want to say no as the GM, but the responsible GM thing to do is to allow it but to have some inherent limitation on it. For example, each time you do this, you weaken the magical texture holding the bag together. It is designed to hold items, not powerful magical creatures that do nothing but buffet the sides and beat at the 'lining' of the bag. Let's say that each hour the elemental is in there, there is a 1 in 12 chance that it deals damage to the bag. The bag might have 5 'hits' before it is ruptured. I might not even tell the players this - but on the hour when the 5th point is dealt, the heroes might be in the middle of a fight, or resting at the inn, or walking across the countryside (they do a lot of that) and bam - the bag explodes (maybe dealing 10 hits to all in a 10' AOE, check reflex for half) and out pops the really angry elemental ready to throw down.
I might even allow mind checks once the bag gets over 3 hits suffered to notice little tears forming on it, or threads coming apart in its seams.
I like this on all fronts - both as the player and GM. I understand that I cannot have 'unlimited air elemental ally' attack for balance purposes, but the GM allows it, with a natural consequence for the choices the players make.
And then I probably make sure they never find another bottomless satchel and they have to carry all of their treasure in regular old sacks like a bunch of scrubs. That'll teach 'em.