Central to the idea of the micro-megadungeon is speed. Trust the players to slow the pace down with roleplaying, exploring, decision making and so on, but the environment you build for them has to be trim and fighting fit to curtail extraneous delay. At the same time, for the locale to count as a megadungeon, it has to be in itself completely sufficient for a whole campaign. Striking a balance between these two aspects in each element of the megadungeon is key.
If the megadungeon is the tent pole around which an entire campaign evolves, the same remains true for the micro-campaign. Thus, while the megadungeon deserves the greatest detail (and its inestimable depth provides limitless adventure), it is not the only celestial body in its cosmos. Rather, the megadungeon anchors many other lesser locales. With a micro-setting, we can limit the environs to the bare minimum, including a Town and three minor adventure locales (one designed for encounters at each level, one through three).
These five destinations should be within short walking distance of each other, to prevent the need to camp outdoors or incur wandering monsters while traveling. If the Town is in the valley, the forlorn monastery (underneath which is the megadungeon) should be just up the hill (with the three adventure spots down the valley, up the valley and on the opposite hill). Wilderness adventures are still possible, but should only occur when the players are specifically seeking them. Furthermore, it may help to confine traveling to within the immediate environs. There may be a bigger world out there, but for the moment the roads are washed out and the characters are forced to stay in this secluded purlieu.
Like the rest of the backdrop, the Town is minimal in detail. Dramatis personae should include an armourer, an innkeeper, a merchant, a liege lord and a priest, each with their own estate. The first three provide obvious services, while the liege lord can give quests and hire out his garrison (10 coins per spearman, with 5 coins per week upkeep) and the priest can Raise Dead for a number of coins equal to the characters accumulated experience points (a flexible way to scale costs to player progress, but be mindful of the survival rates for low Constitution).
Each minor adventure locale should be tailored to a different level, with perhaps two dozen monsters at each site and a quarter their value of experience in coins and treasure. Typical camps might be bandits (600 coins), Gnolls (1200 coins) and Bugbears (1800 coins). Any victory against these enemies will license a similar reduction of monsters and wealth from the megadungeon, to keep adventuring in that locale on track.
The centerpiece of the micro-campaign, the micro-megadungeon is considerably more dense than your regular megadungeon. Ideally, each level should be crammed onto a single page of Michael Shorten's "One Page Dungeon Template," a 30 x 30 square space (with each square equaling 10'). (Unfortunately, "Chgowiz" has decided to pull out of the online community for the time being, so instead I offer up my version of his famous template.) Each level should have monsters of roughly the corresponding level, with enough hit dice of monsters to provide 50% of the experience towards the next level (where 1 hit die is worth 100 experience points). The remaining 50% of experience should come in the form of coins and treasure. Each level should also have 1d3 magic items scattered about. Assuming a party of four characters, our micro-megadungeon might breakdown as follows:
Thus, the first level of the dungeon will contain between 30 and 40 rooms (this is a good example of density), around 40 inhabitants (1 hit die each) and 4,000 coins in treasure. The second level will contain about 20 inhabitants (2 hit dice each) with a similar amount of treasure, while the third level will have around 27 inhabitants (3 hit dice each) with twice the treasure. A crafty party should be able to defeat one stage each game session and reach fourth level by the end of the third floor of the dungeon. The dungeon can go deeper, branch off in areas, grow and develop, of course (in fact, to count as a megadungeon, it must), but with these tightly designed first few levels, your players should be able to get to mid-range fairly quickly. More than anything, this allows you to showcase two distinct stages of Dungeons & Dragons, as fourth level characters are noticeably more competent than first level ones, displaying to new players the different play styles brought through this growth in character power.