I'm estranged from my 'father', so I usually treat my grandad on fathers day! I think its really hard to buy for men, as with ladies its cuddlies, chocs, all the 'smellies' like perfumes/bubblebaths/etc/etc....with men it tends to be the same old aftershave/razors/tie/socks!
This year though I've managed to find the most amazing keyring for him with the word 'grandad' engraved. We'll go out for a nice dinner and I'll treat him to a nice plant...he's obsessed with his garden/plants/trees!