Unexpected pauses can buy time for planning, but they can also be boring for the players and can discourage creativity. Instead, I like to keep a few "pocket problems" in store to slow down the party as needed. These are the kinds of things that you can drop on them any time if you need to take a breather. By the time the party deals with the challenge, you'll be able to comfortably wrap things up for that evening and have a week or three to plan the next session. I don't like random encounters, so I also use this sort of thing instead.
PICKED POCKETS
A band of pickpockets steal from the adventurers in a crowded city. This works well if you make the pickpockets smallish children. I sprung this one on my party, and it led to one of the characters bleeding out in a forgotten back alley while the rest ran away, abandoning them.
While seeking information in the poor quarter, I rolled a few pickpocket attempts against random party members. A few minutes later, I told the fighter his purse was gone. The character grabbed one of the kids, beating him (to the horror of the rest of the players) into leading him back to the Theives' guild. By the time the crying child had led them to the warehouse to meet his handler, the player had lost all the good will of the party and was almost alone. From here things took a turn for the worse when the player tried to exchange the pickpocket as a hostage for his stolen money. The handler, a stubborn man who saw his 'crew' as his children, ordered the band of thieves (hidden among the shadows) to attack. Chaos ensued.
THAR BE DRAGONS
I like me some dragons. I put dragons in all my worlds, and generally imagine a dragon to rule over perhaps a 50-300 mile radius of their lair. As such, I make sure to announce dragon sightings on a regular basis off on the distance, if the party spends much time near a lair.
If I need a bit of time, the dragon can cause some chaos nearby. Setting fire to a nearby farm is a good option, but dragons can do so much more! Maybe the party encounters a band of dragon-slayers, alive or dead. The party could seek out the lair or rescue a town's "tribute". They could be hired by a high-level dragon to do any sort of job, which would explain the dragon seeking them out by reputation. The options are limitless.
TRAVELING BAND
While traveling, the party encounters a band of entertainers who carry ill tidings from a nearby town. This is a good way to force a quest hook on the party if they missed the one in town. They can spend the night with the entertainers, who could number anywhere from 4 (small family) to several dozen (traveling troupe or circus) depending on your world. The party might dine with the travelers, or could merely speak with them and then seek out the relevant quest hook.
A larger caravan could hire the adventurers on as guards, or include their talents as part of their act. The fighter could act as strong man, the rogue as acrobat, the wizard as magician, etc. This can also serve as a way to get the party from one place to another across a guarded border, or to allow them to travel through dangerous land.
Later, the DM can tug on the players' heartstrings by allowing the players to find the caravan destroyed/killed/burned down to introduce a new villain/dragon/band of assassins after one of the characters.
ASSASSIN'S GREED
The party is attacked during the night, and the assassin carries instructions signed by a local lord. This can lead into almost any sort of villain with resources. A good way to spin this is to introduce a friendly NPC and then have him betray the party when they are vulnerable. Maybe their guide in the mountain pass cuts their rope and then flees into the forest. The acolyte leading them into the catacombs below a church could trigger a trap or lock them into a dangerous room. An innkeeper or friendly noble could poison their food, or a fellow thief could turn them in to the guards. In any case, this serves as a setback and that can also work as a flexible hook to a longer adventure.
SPACE PIRATES!
For the truly desperate DM, there are planar pirates. You can rip open a rift and sail a ship through it, literally anywhere, to throw pirates at the players. Like the dragon, these can drop anywhere, and could even seek out the players as a quest hook. The ship can be anything you could imagine, though I personally enjoy Eberron's elemental-driven vessels. The crew could vary wildly among races, and could consist of a anywhere from four to several dozen members.
As an alternative to this, the party could discover a ship or village that had been attacked by planar pirates. This is also a great way to foreshadow a later planar campaign by getting the players familiar with the idea of the planes. To expand on it, the pirates could be fleeing some other threat; like a flying inter-planar riftbeast covered in writhing tentacles and mind-breaking fangs. Much cooler than bandits.
These are only seeds to an adventure, of course, but hopefully they prove useful or inspirational. These can each easily be stretched into something more developed, but I include them as a challenge that would take about 1/2 a session to solve. They can all be dropped almost anywhere with minor changes, and although they aren't subtle, they are also rather varied and should keep the party's attention.
For the truly desperate DM, there are planar pirates. You can rip open a rift and sail a ship through it, literally anywhere, to throw pirates at the players. Like the dragon, these can drop anywhere, and could even seek out the players as a quest hook. The ship can be anything you could imagine, though I personally enjoy Eberron's elemental-driven vessels. The crew could vary wildly among races, and could consist of a anywhere from four to several dozen members.
As an alternative to this, the party could discover a ship or village that had been attacked by planar pirates. This is also a great way to foreshadow a later planar campaign by getting the players familiar with the idea of the planes. To expand on it, the pirates could be fleeing some other threat; like a flying inter-planar riftbeast covered in writhing tentacles and mind-breaking fangs. Much cooler than bandits.
These are only seeds to an adventure, of course, but hopefully they prove useful or inspirational. These can each easily be stretched into something more developed, but I include them as a challenge that would take about 1/2 a session to solve. They can all be dropped almost anywhere with minor changes, and although they aren't subtle, they are also rather varied and should keep the party's attention.