i'd get him a puppy, with some instructions, as follows...
convince the wife to come on a walk with the new puppy, let her hold the lead.. find a nice path along a cliff top, at the highest point of the cliff push her off, this is the clever bit but a little sad as you are sacrificing the puppy... when emergency services arrive just say the puppy made a bolt for it cause he saw a rabbit or a twig or whatever and dragged the mrs over the cliff ...
and there you have it ....the perfect murder .