Saw this on another board and thought it was really cute.
Dear FI,
I know that you have been really stressed lately with moving, work, and graduating. I just want you to know how proud I am of you for all that you do and all that you have accomplished, you are one of the hardest working people I know.
Now I know you don't understand why couples do engagement shoots, but it's really important to me to have them done. So thank you for putting up with the idea and helping me get everything organized and ready for them on Tuesday. Also, if you really must, it's okay to do some zoolander expressions during some of them. Thanks for being such a good sport!
I love you,
Me