The Rubber Hose (stage prop) symbolizes Willy's apparent attempted suicide by inhaling gas. But the ironic symbolism is that it also represents Willy needing that very substance to provide warmth and comfort to his family.
Diamonds is another symbolic representation of tangible wealth to Willy. And wealth, to Willy, being a validation of ones labor/life and the ability to pass material/possessions down to your children. And Ben's discovery of diamonds and his subsequent fortune which also symbolize Willy's failure as a salesman. And Willy 'missed the boat' when he failed to follow Ben to Alaska, ending Willy's chance at financial security. In the end, Ben encourages Willy to enter the “jungle” finally and retrieve this elusive diamond... basically telling him to kill himself for the insurance money in order to make his life count for something. (And that being said, I suppose the "Jungle" is also symbolism.)
I have also seen reference to the tennis racket Willy observes when he chats with Bernard in Charley’s office. It is said that the tennis racket is a symbol of Bernard’s success and Biff’s failure. Biff and Happy wanted to make a fortune selling sports equipment but Bernard (who was on the sidelines in high school while Biff played sports) had the racket.
Finally, others reference the Flute being symbolic. The music throughout the play represents link that Willy has with his father (and with the natural world). Willys father made flutes, and was apparently able to make a good living by simply traveling around the country and selling them. The flute also reflects Willy and the road he didn't take -- working with his hands like his father, which would have been a more fulfilling career for him.