The enthusiast in your life will be happy with any of the upgrades I've suggested over the last 5 pages, from the choices of a GeForce RTX 2060 SUPER, GeForce RTX 2070 SUPER, or GeForce RTX 2080 SUPER -- and everything in between and even higher-end than that in the flagship GeForce RTX 2080 Ti.
If they need a new monitor there are a bunch of options at 1080p, 1440p, and 4K -- even right up to 4K 120/144Hz and above that with a new Large Format Gaming Display or Big Format Gaming Display in the larger-than-life LFGD and BFGD options.
There is a growing list of RTX-capable games that will only continue to grow in 2020 and beyond, with amazing looking games (that are actually super-fun to play) like Control, Call of Duty: Modern Warfare, Metro Exodus, Quake II, Minecraft, and many, many more. That's also just the RTX-capable games, let alone the thousands of games out there and most importantly -- the games you play, will play better on GeForce RTX.
But if you're after something to throw into a 'stocking stuffer' (digitally, I guess?) then you could grab Quake II RTX for just $4 on Steam or even Metro Exodus' new DLC which is $8 -- that is, if you already own (or buy) Metro Exodus. Minecraft is available for less than $30 and has a nice RTX upgrade, too.
There you have it -- a guide to what to buy the enthusiast in your life. I don't think there'd be a gamer in the world that would be unhappy with any of these gifts, especially a shiny new GeForce RTX SUPER graphics card or some new games. If you've got deeper pockets a Large Format Gaming Display or BFGD would need Santa's actual personal sack to fit it -- but boy would you get tears of pure joy from buying a loved one a present like that.
Albeit, you wouldn't see them again -- they'd be playing games for the next few years with a huge smile on their face.
Happy holidays, everyone!