Awwwssss.first of all..so cool you have a hubby that you can do that for...is lovely to see someone saying such nice things about their husband. I'm not a very crafty person, but something like a "Dad's day scrapbook" might be nice. You can put photos, mementos and other things in there that are "great dad" related. (As they pertain to your family.)
I know with mine, its hard to do the same because he's a great dad and there's no thanking him for it..he just is and was raised to believe that everyone pitches in. His attention to our children is wonderful and he is even a good "fuzzy dad" and he had no pets growing up.
So I suggest something that memorializes and looks ahead at his "great dad" days. Make it something that you can look at fondly throughout your kids' days, if you have more you can add from them, etc. Eventually you will have a tradition of adding to his collection of memories, and each year you get to look at all the wonderful things from years before.
Good luck, and again, its so wonderful to see someone saying "My husband is great and helps me out". We don't see enough of that these days as it seems its always easier to "blame the man" then to thank him. I wish I knew a specific to tell you but my hubby is horrible about giving me even the slightest of ideas. He says "he doesn't want presents" and we do have a "no gratuitous gifts rule" that says basically nothing just to get something. We don't always have something to give but our time with each other so we take advantage of that...which if you are looking for something personal, you might go that route too. Places like Glamour Shots made a mint on that idea a long time ago...now with the internet age and lots of freelancers out there, its possible you don't need to go to a studio..) So if you are into that...grinz..go for it:) I am a photographer so personal photos are something I can suggest...but do be careful if you go that route and meet your photographer before hand to make sure that this is what you want. Always take someone with you to the shoot you can trust (since this would be a gift I guess hubby is out but take a girlfriend). He might appreciate a very feminine gift from you....sometimes we forget our hubbies need to feel "wanted" too...grinz. We just assume they know I think. So maybe something from "junior" and something from "wife" would be nice instead of from "mommy"...hmm...I kind of like that idea...might do it meself :)