The biggest trouble with trying to make something up off the cuff is finding a way to provide something interesting and creative. Usually, preparing an adventure involves drawing out a map, placing defenses and traps, and at least mentally anticipating how the monsters will react defensively, regardless of how much you actually write down. Unfortunately, this kind of thinking takes time and relies on thoughtful preparation. So how can you make something quick that still delivers?
Put the characters on the defensive. Generally, most adventures involve the PCs tracking monsters to their lair, overcoming their defenses and carting off the loot. For a change, bring the monsters to the PCs, and make them do the defensive preparation and strategy as the game plays.
Defend the Stronghold - The stronghold doesn't need to be a fully plotted castle with moat overlooking a broad countryside, it can be an abandoned cabin in the woods, a knoll in the forest, or anything similar. The key to the situation is that they are the defenders. They need to assess the situation, choose a defensible position, and ration their resources to withstand the assault. My suggestion is that they defend themselves through the night or day, long enough to provide multiple sallies against them, but some period with a definite ending. This will help in parsing resources.
One beauty of this setup is that it allows players to see some of what goes into designing an adventure, and possibly inspires them to create their own to DM.
Another benefit is that since the situation will be more challenging to process, the monsters themselves can be very generic. Run some orcs and goblins at them, maybe some wolves, or a bear. With three or four basic fights, you'll have enough to take up an evening, and keep everyone entertained. Instead of time between encounters taking characters from town to town, up mountain paths, or so on, they can spend time healing, making forays into the woods for firewood or water, searching outlying buildings for lost supplies, healing potions, or a better spot to defend. It might be worth loosening some of the rules about long rests to allow an extra healing, or spell recovery. I suppose you could say they were defending for a longer time with fewer attacks per day.
This is a "lite" encounter night, so there is more to gain from relaxing time constraints in order to get a few hours of play in than being a stickler for going "by the book" and sending everyone home because you have nothing prepared, or the first two fights exhaust the party's resources. Let the creativity and non-combat elements of play come from this risk, and give them options in scavenging to make up for what they need.