food - feed them to death - works especially well on people who have addictive personalities - they just get fatter until they've ruined their hearts and other organs and then they weaken and die. I'm quite sure its been done as we speak by vengeful wives and husbands all over the place!
Well, one could always use a aeromatic nailer. I know it is considered a gun by some, but not in the technical sense.
If you have a big microwave, and I mean a BIG microwave, you could use that. They claim it fries things from the inside out.
Cars have been known to do the trick, but are very messy.
Drug over does seems to be a favorite amongst those who want to leave no trace.
Of course, there are the non weapon routes, like drowning, shoving someone off a cliff, suffocation -- the possibilities with that are endless -- knew a man who died last year in a wheat truck. Allergies can be a killer -- keep cats away from me, or one of us is going to die!
Plus, there are natural methods like hail, lightning, meteors, tornadoes. They are harder to stage, but if you are looking for creative...