Sort of a journal activity, in the list genre.
What makes Dad, Dad? That is to say, what's special about your dad? Some ideas:
What is special about Dad?
What do you love (or like) best about Dad?
What does Dad do that makes you laugh?
What is Dad good at?
What is Dad bad at? (as long as this is affectionate)
What do you enjoy doing with Dad?
What has Dad taught you to do?
Go for 10 sentences.
Brush it up--check grammar, spelling, etc.
Make it pretty--make a card, write it in calligraphy, type it on the computer and play with fonts and colors, draw a picture.
Happy Fathers' Day!