My K-Cup machine sits next to my SodaStream machine, so guess what I did?
That’s right-I made a large cup of blonde roast coffee, let it chill, poured it into my SodaStream bottle and carbon-dioxied the shit out of it. It don’t taste half-bad, btw.
What have you done with your Sodastream (or equivalent)?
Hmmm, carbonated coffee…not sure how I feel about that, but I’ll have to try it!
I haven’t tried anything fancy with my SodaStream, but I do sometimes make my own ginger ale using the carbonated water. You can find various recipes online for making your own ginger ale (it’s really simple), but here’s how I do it: I boil pieces of fresh ginger root in a pan with regular water and vanilla - put on simmer for about 15 minutes or longer after it comes to a boil, and if I want a sweetener I’ll use Stevia or touch of honey and mix in. Then I strain the ginger root pieces out and chill the “ginger syrup” in fridge to add some to a glass of carbonated water. Or if you want a bottle’s worth, just put the ginger juice in your bottle with water and carbonate away. I don’t know the ratio of the juice to carbonated water…I just go by taste as to how strong I want it, and that can vary.
Doesn’t Sodastream still strongly warn not to carbonate anything but plain water, or has it changed since I got mine years ago?
“Born to be wiiiild…” 
You rebel!
BigCliveDotCom has done a handful of videos putting various (drinkable) things in his soda stream. Generally liquor or wine.
He has a recent video where he weighed the liquid before and after being carbonated to see how much CO2 it absorbed.
Hydraulic Press Channel did a few videos on it as well, but using really odd things like pureed (solid) food. I don’t think those turned out as well as Clive’s.
I mean, there are tons of things you can do if you want weird drinks. I’ve been making my own syrup for the Sodastream for years, and while it doesn’t save any money over purchased syrup, the quality tends to be better and the flavors are customizable to your tastes.
I’m sure everyone who has one has also seen the note that almost any commercial syrup (sno-cone, coffee, etc) can be used to make drinks less expensively than using the bespoke syrups, and as an example, any vanilla syrup makes a fully functional cream soda.
I had a friend that, somewhat like @Czarcasm, made carbonated coffee, although they added Via (mixed with a little hot water and sugar to be a simple syrup) which they liked, but never bothered to do the same since I’m a milk-in-coffee person.
My wife is all plain carbonated water all the time, and I just use it to make a two day supply of cola for Rum and cokes. 
Pretty sure I saw a video where a guy tried to carbonate red wine with a SodaStream. He ended up with wine-colored walls in his kitchen.
Definitely not recommended (at least with normal sodastream syrups) - I got Pepsi Max all over the walls of the kitchen. Was not popular.
I make a 50/50 mix of Pepsi Max flavour and sugar-free Raspberry/Cranberry. It’s my goto soda.
Since BigCliveDotCom has been mentioned, here’s TylerTube,
NileRed burned some diamonds in pure oxygen, collected the resulting CO2, and used that to make some really expensive soda water.
