There are some father's Day Cards here:
Free Father's Day cards and cute printable cards, Father's Day greetings, and cards for Dad
Those aren't for coloring. They are designed to be customized online and then printed, but you can see how the cards are made. If you still want to color, just have the students make a quarter fold card, like those on the site above and they can design the front and write on the inside.
There are also printable envelopes on
123 Print Cards, so you might be able to use those as well.