Years of service awards are about more than tenure. You must treat it more than just a 'participation' award. This is especially true for someone who has spent such a significant portion of their life and career with your company. For a 20 year celebration, employees want to be appreciated for their talent, their value, and their accomplishments during their time with you.  They also have a wealth of knowledge and experience to impart on younger employees. This means giving them a unique celebration with personalized gifts and team accolades is key. Find something they thoroughly enjoy and will find useful and is personalized to them.