I've One-upped all of the answers that are already here.
You have two issues, how to create an adventure quickly, and how to keep the workload down.
Pregenerated characters are a good idea.. BUT you may want to get the PCs to build them just to save your own workload. To compromise- give the PCs some guidelines: "you are all playing dwarves" or "Arcane power source only" or "Just Essentials PCs" is a good way to get everything focused from the start. You might say "two of you are strikers, one of you is a leader, and one of you is a defender. Your choice which is which. This guidenace gets the players started about what kind of character they should make if you decide to go that way.
Both the Delves, and the Encounter group type deals are also good calls. If you have access to the Adventure Builder/DDI, it's also pretty easy to shoot out some printouts of monster blocks and recombine them into encounters according to the formula that an encounter budget is equal to
PC-level level in monster XP (1st level =100xp etc) * number of PCs
+/- Give or take a few levels of bump.
So if you print out a single kobold slinger, kobold dragonshield, kobold miner (minion) and kobold wyrmpriest- you can turn that into 4 (or 40) different encounters just by altering the mixup.
This is a standard Dungeoneering Adventure Plan. There's nothing amazing about this, but it does work, especially and as long as you remember that the "story" is what players and their characters do, not anything you plan for.
Grab a big stack of dungeon tiles or just creative with a battlemap and markers.
Improvise a reason for adventurers to take on a monster-lair. Even just "there's treasure there" or "they are kidnapping people.."
As the players explore- describe, improvise the lair room by room. It could be 3 rooms or 30 or 300. But there are only 2-4 encounters you need to run. Be sure to improvise terrain and atmosphere! You might even draw dungeon tiles randomly from a deck.
Whenever you get to an encounter, drop in an improvised mix of the monsters you chose that somehow approximates the encounter formula. Be sure to improvise terrain and atmosphere again!
Plan 3 or 4 treasure drops. Some of the treasure might be unguarded, trapped, hidden, or inaccessible (for example, a ring at the bottom of a deep well, or a chest behind a closed porticullis gate).
Consider putting at least one skill challenge (perhaps a complicated puzzle or trap) and at least one NPC encounter (in a dungeon perhaps this could be an encounter with prisoners, a ghost, a fellow adventurer, a crazy outcast kobold..or anything else).
No story planned, improvise everything. Let the PCs explore the characters, and do whatever they like. Go with the flow, play off what the PCs do.
Example of an Improv 4e Adventure:
If you have 4 level 1 characters, that's about 400xp to work with.
early in the adventure you might use 2 dragonshields (thats 250xp) and 2 slingers (200xp) that have set themselves up with some barricades and such. Guards.
Later on, after exploring several rooms, the kobolds ambush from a room with overhanging ledges. Use 2 more slingers, a wyrmpriest and 4 minions.
Later on the PCs encounter a group of enslaved miners, whom they rescue. And the miners tell the PCs about a dragon-idol the kobolds are forcing them to excavate. Roleplay roleplay. Invent NPCs. Do it on the spot if necessary.
Later on the PCs might come across the dragon idol itself, which is magically unstable, intelligent and able to collapse part of the caverns unless the pCs intervene via religion, history, arcana, and theivery- to calm it/remove it's magical residuum core.etc. Skill challenge.
Improvise some tracks or trails that lead through the kobold warrens to a final lair.
Wrap up the final encounter with a wyrmpriest, a dragonshield and 10 minions and the rest of the treasure.
To make it less of a railroad, generate the encounters beforehand (and create 2x or 3x as many) and create a map and matrix. That's really not that much more work, especially in 4e.
First, set up a Conflict Web. Start by setting up your factions that are involved, and why they are competing/conflicting. This is more to give you a set of motivations for any given group, leaders, etc. and allow you to simply improvise based on the group's needs/ambitions.
The Conflict Web is not static, it's a starting point. So you may easily see characters shift alliances or make temporary truces to accomplish goals.
Second, once you situate the PCs into the scenario, look at their goals, and likely problems they will face in terms of Logistics and Politics. This is effectively similar to how Apocalypse World produces "Fronts".
After each session, look at what the PCs attempted, who was affected, whether any NPC groups made major moves and figure out who is going to react and how. You can choose to update either the Conflict Web or the Logistics & Politics list, though I usually find myself only having to do serious updates after 3-6 sessions because it's relatively easy to track what happened with simple notes.
Both of these tools can scale up or down, so you can do intergalactic empire politics or the 28 guys stuck in a prison together, based on whatever fits your campaign.
Best Answer
One simple way for sandboxy games: Explain to the players that they need to get back to base by the end of the session. If they don’t, then you’ll roll on a table you’ve made up to determine what happens on their way back to base.