Fathers are much more easier to please than mothers. No need to stress yourself out over impractical things like flowers. Get them a six pack and spend the afternoon chatting with them in the backyard would be more than enough. If you want to step up your game, give them a box of poop chocolates. With the right combination of poop chocolates and corny jokes, it'll surely be a poopful punny day. After all, where else did you think dad jokes came from?