I'll say this at the risk of sounding like an unappreciative jerk - but please realize that it isn't intended that way.
Don't buy them "things".
The problem with buying something for "someone who has everything", is that they are notoriously picky with their gear. Trust me - I'm one of them. If I mention [to my better half] that I'd like a widget, there is a 99% probability that I've done a bunch of research & have picked out the specific brand/model that I want. Anything else will still leave me wanting the specific one that I had already decided on.
Unless they give you a manufacturers part number - it's unlikely that you'll get what they *really* want. Sure, they may like it, but they'll still want the brand/model/size/what ever that they really wanted to begin with, and will eventually buy that one anyway. That doesn't mean that they won't appreciate the gift - but it won't be what they really wanted.
Consumables & experiences, on the other hand, make fantastic gifts. They are there, they get enjoyed, then they're gone. No one gets stuck with something that they didn't really want to begin with. A nice bottle of whiskey, some good coffee, a subscription to a magazine, a years worth of netflix, a nice dinner out on the town, etc. You get to spend some time with that person, have some fun, no one gets stuck with an ugly sweater or another dust collector.
good luck!