Years ago I worked for a large multi-national company.  They celebrated everytime a site achieved a million work hours without a lost time accident.  At this point every employee working on the site received a safety related gift.  These ranged from home escape ladders and fire extingushiers through to items such as mugs and tools to cut through your seatbelt if you were trapped in your car.  These as you would expect did not please everyone but they did serve a purpose in the short term.  In the end it got harder and harder to find a suitable gifts.  I think in the end they started to give a financial gift to local charity and did some publicity. 

In their heyday, ICI used to hold raffles using the one million hour criteria for things such as holidays in the the states and cars.  That led to people being brought into work following a work injury and the shift doing thier work for them and it even drove the reporting of accidents underground. Not their finest idea.

Personally I would rather keep it small and inexpensive have a staff gathering with some food and refreshement and get a director or senior manager to say a few words of congratulation and the expectancy to carry on working in a safe manner.  Additionally some kind of sign to publicise the fact around the workplace.