[NAME, COMPANY AND ADDRESS, ex.
Jill Smith
XYZ Inc.
1234 First Street
Suite 567
Anycity, Anystate  85245]

Dear [NAME, ex. John Smith], 

I just wanted to send you a short not thanking you for taking care of my files while I was on vacation.

I know how stressful your own workload is, and I can appreciate that by carrying my additional workload, you probably had very little free time last week. 

Again, I'm very grateful for what you did, and I would be happy to reciprocate whenever you feel you need some time off.

Thank you! 

Sincerely,

[YOUR NAME, ex. John Jones]