Yes the ds won't know exactly, but even very small children love joining in on making, shopping or wrapping and 'taking things to daddy', besides the fact OP wants to bother 
A card with hand and footprints on? I did this with my then 18 month old when it was Father's Day. Child enjoyed Dad loved.
A book mark? You cut out, punch hole, attach ribbon, let little one put on some stickers.
I've got dh a photobook with about 30 photos taken of the kids, a couple with him and one with me, over the whole of the year, a couple from each month. I got them off our phones, ones that might not ordinarily get printed ifyswim, not necessarily from holidays or big days out, just little trips to the park, playing games round the house. You can really see them changing. That's his gift from the children this year. Cost about £6.50 from Tesco.
Schleich animal models, or model vehicles. Game they can play together - puzzle or remote control something. Superhero Duplo?
A little manbag rucksack for when they go swimming together?