I used to be so good about putting a little note into Benjamin's lunch box with encouraging words or just a heartfelt sentiment. This site called Positively Proud has inspired me to start doing this again for both of my kids so that in the middle of their day they they get a little special love boost from mom.
You can also make your own from a sticky note or recycled greeting card front.
Here are some wording suggestions:
Have a great day!
Looking forward to seeing you soon!
Keep up the great work!
You are special to me!
Do something nice for a friend today.
You put a smile on my face.
Roses are red...
I love you more than...
Inside is an extra special treat for you. (Attach a little candy)