I think I shared this photo that I made for my husband last year for Father's Day already (an idea I originally got from Pinterest), but I thought I'd post it again in case you need an idea like I do every year.
It's super easy to make... I just used an exacto-knife to cut the letters from a piece of poster board, and took a picture of the kids holding it on a sunny day. You don't need to write it out backwards or upside down or anything... but they will have to hold it that way. Also, try to take your photo in the middle of the day when the sun's rays are closer (but early enough that it's not directly overhead-- I had the best luck mid morning). The silhouette will be crisper that way.
This is another photo gift I did a few years ago with my friend Alissa. She had the idea, and showed up with the letter's all painted and everything. All I had to do was take the photos and frame them. Super easy. It's kind of self explanatory... just paint some cardboard letters and have your child hold them. It's a good idea to have them sitting if they are busy as my kids are, so you are shooting from the same perspective each time-- the letters will be about the same size in the final shot. I love how it turned out. You can do "Grandpa" too, just make sure you do 2 different shots with the "D" and "A" and you can spell Dad and Grandpa.
What are you doing for Father's Day this year? When my kids are old enough to initiate their own way of celebrating their dad on Father's Day am I off the hook? Cause I'm running out of ideas!