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Are your players already into the campaign, or are they just starting? If they are just starting, give them an opportunity to learn about underground hazards before they head over to the WHISPERING CAIRN.
It might sound cliche, but kobolds might be just the sort of thing that would have shut-down mine operations. They tunneled under the office and gained access to the basement. From there, they stole weapons and equipment. They weakened support beams in the mines to create death-traps for the miners. They may have even used the basment entrance to sneak into town and make-off with a few tasty miner's children.
Perhaps old Ulgo Fant developed a repuatation for having a deadly mine as a result, and his business failed because no one would risk working for him, even Smenk's operation was a better option.
When the characters go down under the office, they find a network of small tunnels. Size Medium characters find that they have to stoop and can only use short, thrusting weapons within their confines.
The kobolds have mostly left, but their traps remain, as well as a small group (maybe 6 or so) who felt that there was still some good hunting in the area. Once they are aware that the characters have entered their lair, have them lead the characters into traps, ambush them from side passages, or try to cave-in passages and trap them.
Don't be too brutal, but make the players aware that going underground is serious business, and being prepared to fight underground is no simple thing.
There you go, nothing fancy, nothing terribly original...have at it.
I made a short side trek for my campaign called The Cellar Door (click on it to go to the archive).
It's a haunted house-style adventure for 7th level characters, but easily adapted for lower levels. Great for a Halloween diversion, as well.