Dimming interior lights isn't really that unusual when you put the battery under load. It's more noticeable on some cars than others, but many I've driven (Honda and other) will do that. If it's a huge difference in brightness, than I'd look at the condition of the battery (what it puts out w/ out engine running) and the alt (check what battery puts out with engine at warm idle) with a volt/ohm meter.
May also be more noticeable w/ the engine off as the battery doesn't have the help of the alternator.

Your ground cables are the ones that run from the engine or transmission to the chassis. As well as the negative battery cable and a few others such as the one at or near the fuse box. No matter how bad they look, as long as they aren't loose or full of corrosion at connection ends, they shouldn't need replaced.

If hitting it consistently brings it back to life, the temp. gauge issue has to be a problem with gauge itself or the cluster's connection.

edit: after reading what Shounin posted, I'd agree to make sure the battery terminals look good too. And removing the IACV (when engine is cold), cleaning it with carburetor cleaner (after removing rubber gasket/o-ring), and putting it back on to the proper torque is a good idea. Fairly easy and it often fixes idle issues. If not, it may be the EGR valve, just guessing.
Good idea to wear safety goggles, etc. just in case when using carb. or brake cleaner.