• Personal notes. I would venture to say that everyone welcomes sincere words of appreciation.  Handwritten messages of congratulations from supervisors and corporate leadership and/or "Thank you" from the staff and clients or students at the agencies or schools where the employee volunteers would say it best.     
• Company-wide acknowledgement. Stories, profiles and/or testimonials from or about volunteers and their projects in e-newsletters, intranet boards, social media, websites...any means of communication that can get the word out throughout the workplace -- you might even include suppliers and customers.  Do keep in mind that some people are private about their volunteer activities, so it may be a general "Way to Go" message with a few specifics rather than identifying people by name.   
• Celebration activities like Dress Down days, pizza / cupcake / taco parties can honor volunteers as well as bring employees together.   
• Reporting.  Highlighting benchmarks and the actual work that has been accomplished by your employee volunteers.
• Tokens.  Certificates, "Proud member of the ABC Company volunteer team" pins, company-branded items or t-shirts.  These will be worn or displayed with honor.   

Remember that any celebration of volunteerism - individuals, teams or projects - is a morale and team builder as well as a celebration of the culture of community engagement in your workplace.  It's a win/win/win! 

Questions about workplace volunteerism?  Ask Yvonne!