my dad is the hardest of all people to buy for...so i got him flying lessons over the area were we live as it was something that you could never experience in an areoplane......she was stunned by it, it was brill!
For my dad's last birthday, I signed him up for chocolate of the month club (dark chocolate only.) He really enjoyed that, since every month, he received a lovely package of chocolate. And they have these for just about everything nowadays.
When my Dad was alive - it was a case of what do you buy the man that has everything so I often just gave him an up to date framed picture of me - his little girl!!!! sometimes a pic with my brother sometimes including our partners & children.
How about naming a star after him, or buying him a title! These are defiintely more interesting and fun than a pair of socks! i think you can get the name a star on www.nameagift.co.uk