“Kobold Hall”, the short adventure at the back of the DM’s Guide, has potential beyond the mere five encounters in a row.
Firstly, change the dragon at the end from white to green. In the first chamber, make the two squares in front of the glowing pool a slide trap (trap-loving kobolds do live here, after all). Describe some sparkly lights shining at the bottom of the pool to lure a PC closer; if the PC steps in one of the trapped squares, into the pool the PC goes. While in the pool, the PC takes 2 poison damage. Same rules for getting out of the pool. Once the PC is in the pool, the sparkly thing is either revealed to be an illusion, or is worthless—but shiny—junk. Or, maybe it is actual treasure.
Also, turn off all the lights. Kobolds have dark vision, and don’t need them.
The kobolds should appear sickly (but are still at full power), because the dragon they serve breathes out poison.
Each subsequent room is lower than the one prior (there are stairs leading down in each room), so the atmosphere grows colder, maybe, as the PCs progress.
The air quality also gets worse—a faintly “off” odor in the first room grows increasingly repulsive the deeper the PCs delve, until they are positively choking. Perhaps have the PCs run across the dead body of an adventurer (someone they know—or know about) lying on the floor, clutching at his neck, his skin a mottled green and black.
Kobold Hall used to be the residence of a lord of Nerath. Why were these lower levels built? There is a room with caskets, so maybe these were burial chambers. Add the odor of death to the poisonous vapors.
Maybe have murals and frescos on the walls, that are faded and possibly be defaced. Broken furniture, that sort of thing. Names on the caskets. Elements to show that this place wasn’t built so that one day PCs could kill kobolds here.
Have the PCs find prisoners tied up. Maybe the prisoners are missing members from a merchant caravan that disappeared a few days ago. Maybe they are bandits that worked with the kobolds awhile, but were betrayed. Maybe do both: the bandits captured the merchants, expecting to be paid by the kobolds, but found themselves taken prisoner, as well. The merchants were taken deeper into the dungeon.
I would introduce an animal of some sort—a half-starved dog that follows them into the hall, for example, and helps them out as they explore. It stops suddenly, and the hairs on its neck raises up as it growls menacingly at something beyond the reach of the light. It seems to take to one of the PCs particularly. In the end battle, if the PCs are winning against the dragon, in a final act of meanness, the dragon cracks the dog’s back and flees out a secret exit. The exit slams shut, preventing the PCs from pursuing the dragon. They gather around the whimpering animal, that looks soulfully into the eyes of the PC it liked, as it licks the PC’s hand. It dies with a sigh.
Now, you (possibly) have a recurring villain.