So, as an artist I know all the things I'd love to get as Christmas gifts, and I'm sharing it with you, in case you have an artist in your life, who knows, it may give you ideas :)
An iPad Pro, would be first on my list. You can draw and illustrate amazing things with it, and even if it is someone who works traditionally (with real paints, for example) it must be something really fun to dive into. You won't believe your eyes the things you can do as an artist with an iPad.
One Watercolors set, from Windsor & Newton, for me the best watercolours in the world, I just love their pigments, their tones. It can be a small one if it would be a first time in watercolours, or just look for the biggest one you can afford.
It can be a portable version, in their COTMAN series (student quality, beatiful and cheaper) like this one. Or an amazing big set like this one, it depends on your budget.
Moleskine notebooks, for sketching, or for watercolor sketching. Those are simply amazing, their paper is dreamy for working on, with any material you love. Just pay attention if your artist uses a lot of washes, watercolors and humid materials so you get the Moleskine for Watercolor. (Look this one for example)
This requires some Research on your part, If you happened to know what other artists he/she admires, it could be a really cute and way cheaper gift to get if possible (by searching for that artist's website, or own online store; and it would depend of she/he has one) a mug illustrated by him/her. Also it could be an art book if there is any, from that artist.
Any fancy professional set of markers (COPIC ones for example) or graphite pencils, or watercolored pencils... search for brands like Windsor & Newton, Koh-i-noor, Faber Castell, Caran d'ache, among others, or get advice from the salesman, but ask for pro tools.
An easel, a nice one. Again, it depens on what your special artist already has or hasn't. Take a look first ;)
Usually for a painter, easel H frames are good ones. Here is an example that I really like. Not sure it should have wheels as I'm not sure it would be moving a bit when I use it, so there you have one that has no wheel haha.
I would give you a link to the one I have, but I made it myself so...but is an h frame easel. That, if you are thinking in easels to put in her/his studio.
All kind of notebooks, are something that usually artists tend to like.
Something related to things that help inspiration mood. Good music, good coffee, a gorgeous blanket, but here you should really know your artist and how she/he gets inspired and what does she/he need in the studio.
Tools! If your artist loves crafting a lot too, there are amazing tools, like a Dremel Tool kit (or same but cheaper with less accesories here) that can be really useful in case of doing small size sculptures and buildings for example.
The key for me, is that tools, materials or art from someone he or she admires are awesome gifts to give to an artist this Christmas (or any time during the year for that matter lol)
You could also see what needs to be improved in her/his working space. Is is too hot in summer for working, too cold in winter, does she or he needs a better or larger working table? Does she or he have a good lamp? or chair?
What can you do to improve her/his working spot? whatever you do in such area will greatly appreciated by your artist for sure :)
And one last thing, you could also choose a gift that's for example, a different medium from the usual medium your artist works with. If she or he always paints with oils, you could give her/him a watercolour begginer's set... choose an ARCHES watercolour paper block, one small Windsor & Newton set of watercolours (in the COTMAN series for example) and two or 3 big rounded brushes for watercolors.
Personally, for example, I'd love to get a small begginer set for Needle Felting like this one and an introductory book to do some cute needle felted animals... so, if your artist is a curious soul and gets excited about everything, then, find something new, a new technique and go buy a begginers kit :D
Some artists have a passion (like me) for technology also. So, check it has a good phone, a good camera, or a good printer/scanner... but I won't go on too much detail here, because that would need way LOT more research on your side. Artists are usually very picky on what they want and need :)