Dear {Volunteer},
I just wanted to take a moment and say thank you for sharing your time and talents with {name of organization}. I know it's not always easy, but you always come in with a smile on your face, ready to take on whatever is asked of you.
Thank you so much for your help. {Name of organization} wouldn't be what it is today without awesome volunteers like you. Keep up the incredible work!
Cheers,
{Your Name}