The temp gauge in the new Sidekick jumps around quite a lot - is there a cable or sensor easily checked for a loose connection without ripping the dash out?

The first place to check is the connector on the temp sensor.  It is located directly below the thermostat at the front end of the intake manifold.  There are actually two sensors - one for the dash gage and one for the ECM.  You want the one that is closest to the front of the engine near the radiator.  Pull the connector straight out.  Do not twist it as the connector tab will easily break leaving you - well, screwed.  Check the spade connector for corrosion or looseness. 

If dirty or loose, try cleaning it with a bit of sandpaper or vigorous use of a small toothbrush loaded up with something like Lava liquid soap.  Rinse both connectors clean and re-install.  The wire end connector may need a slight squeezing to tighten the connector.  Again, don't go crazy with the squeezing and tugging.

Hope this helps!