Symptoms: Hard starts or rough idles

Spark plugs, well, spark the combustion in your engine. If the spark plugs are all gunked up or misfiring, the engine won't be running and starting properly. This is where the deep sockets come into play (even more so, if they're magnetic). Pop off the covers, unscrew those puppies, and put some new, clean ones in there. Unless the engine is extremely far back in the bay to the point where you can't access the plugs without a lot more work, this is something you can do relatively easily.