My Dad enjoyed having a Eurosport player added to his Amazon Prime, as a gift.
My sons likes mini but good quality toiletries for when he travels for work. He particularly likes the small Molton Brown bath/shower gels. They have them in the MB shops but also John Lewis.
We've bought some books about such as biographies (Elton John etc.) and Paul McCartney's one about his lyrics that's new out, for someone whose general reading tastes we don't know, but love of vintage records and 60s-80s music we do. We also bought some books about interesting maps, popular science and medicine titles, and other new-out biographies, this Xmas and over the past few months for birthdays in the autumn/spring.
An IOU for a specific or non-specific theatre, music or other trip?
Ikea (or John Lewis or whereever) voucher or IOU for a new mattress or something that he'd really benefit from but doesn't want to spend the money on.
National Trust (or NT for Scotland), English Heritage, Cadw, Kew Gardens or some other local membership (joint so you can both go, some allow you to take guests as well, or children/grandchildren).
Cooking lessons. Advanced driving lessons, off-road experience in own car or theirs, or those skidpan ones:
www.trackdays.co.uk/driving-experience/skid/