like a book on brewing or other kinds of fermentation!
Something that connects with their expertise but is new to both of you.
If somebody is into knives, maybe gift a course on blacksmithing or some japanese whetstones.
I bought a set of diamond sharpening plates this summer after having had a cheap Arkansas stone for several years.
Night and day difference.
I can actually get things sharp quickly and consistently now.
Since we're on the topic of gifts, if someone out there is thinking of a sharpening setup as a gift, can't recommend diamond plates highly enough.
I bought a 300 / 600 / 1200 grit set off Amazon for I think $100
Woodworkers might be interested in the Norton water stone starter set of 2 combo stones 220/1000 and 4000/8000. The 220 wears out of flat so quickly it's useless, but I have the diamond plate anyhow. The 1000 and 8000 are great. The 4000 is a little soft, and you'll replace it before the others. Personally, I'll be replacing it with the Shapton equivalent in another year or two.
They're also great on blades. You wanna look for DMT / Dia-Sharp. All the big online knife shops should have you covered here (epicurean edge, etc)
But I've also been given knives a few times, and I loved most of them. It's not like you can have too many knives.
I'm not sure what I'd do with an extra set of whetstones though.
For knives, a nice grind stone or knife oil could be useful.
I think splurge hobby consumables is a great way to go.