Dear {Employee},
What a crazy {couple of weeks/month} it's been, huh?
As you know, we're usually understaffed around this time of the year, so I really appreciate you putting in the extra hours. Despite several people being sick or on vacation, our team was able to meet all our deadlines thanks to you.
Enclosed is a {monetary amount} gift certificate to {store}, which I know you frequent, as a token of gratitude.
Thank you again for going above and beyond.
Sincerely,
{Your Name}