Honestly, shrinking the tunnels down all by itself should be all you really need to do. Only specific races would have any reasonable ability to traverse these areas at all. No human warrior is going to be useful if he has to spend the entire adventure fighting on his knees to be able to shuffle his way through the low ceilings. Assuming they arent short enough to force a full scale baby crawl posture. On the other hand, there probably ARE adventure races that can manage the short tunnels, and larger areas to work with means more room for extra traps and ambush locations. :p

For amusing and random trap ideas. Create a number of pointless (to you) tunnels. Put them on a fairly steep upslope and leave copious sign of kobolds using the area. When they get halfway up the tunnel, the door behind them is slammed shut and locked from the outside, at which time an indiana jones style boulder drops at the top of the tunnel. Just small enough to fit in the tunnel without getting caught, too big to dodge around in any way. Locked door keeps them from escaping and they get smashed.

Pit traps with water and a large slab of something heavy dropped on top once they fall in. Drowned.

Lots of strings for rogues to spot and "disarm" They dont attach to any trap, they are tied to a bell a couple tunnels over so when they get cut or otherwise moved, the kobolds are warned that its time to activate the actual triggers for whatever traps you like.