Here's an easy one you can do with a couple of markers and a small stretched canvas you can find at Michael's, Hobby Lobby or any local craft store.
I used alternating colored markers on the stretch canvas. I outlined the letters for DAD and then had the kids come up with words to describe their dad. Then I wrote the words to make the shape. I made sure the kids wrote their own names and put the occasion and date on the bottom. (I hate when you find things made by kids, but don't know what year it was.)
Another gift dad really loves are the homemade coasters.
So easy! You use plain absorbent stone coasters, some phones printed on paper and modge podge. We didn't try photos printed on photopaper because I'm not sure how well the modge podge works on that type of paper. The one mistake difficulty we had was getting the head shot close enough and still being able to cut a complete circle to fit the coaster. You can see that my kids didn't succeed in that task, but I still love how they turned out. The modge podge mades them nice and smooth and completely water resistant.
Last year we did a barbecue theme (we aren't terribly original around here). To add to the implements, spices and apron, we made a tray to carry items to and from the barbecue grill.
You can get a plain unfinished wood tray at Michael's or Hobby Lobby. Using scrapbook paper and modge podge, we decorated the inside of the tray and then painted the rest. I made sure to put a clear epoxy or shellac coat over everything to make it water resistant. This project was a little more tricky for my 10 year old and we did struggle with the seam on the paper (you can see it above). However, the fact that homemade things aren't "perfect" makes them the most precious gifts because it shows how much the kids worked at it.
The last one we wanted to make last year (and might this year) is a valet tray made from varying sizes of small boxes. We found a set last year at the Container Store. You can again use scrapbook paper to cover the boxes and then glue them together to make this configuration. As a sharp touch you can use the "file markers" to identify the various compartments. You can find the metal frames for those in any scrapbook section of a craft store.
Good luck with your homemade treasures and Happy Father's Day!