A basic flavored air pen like the Füm ones. I didn't feel like spending $70 on one so designed my own. You can put a bit of a cotton ball or the end of a q-tip into the end, and put a few drops of essential oils on it, then you're good to go! I personally love peppermint.
This is also much more economical than the Füm pen, as (at least where I live) 15mL of peppermint oil is around $5, which should last you around 150 refills (assuming you're doing 2 drops per filter, which will probably last you about a day each) which is much better than the $10 for about a week's worth of Füm filters ($10 per pack, which includes 3 filters that last 2-3 days each)
To adjust the amount of flavor (doesn't really work too well but helps if you're not getting enough airflow through) you can move the grip up or down to uncover some of the holes, the more openings the more airflow (and theoretically less flavor)
This is a very basic model, and my first in a long time. I'd love some feedback, and especially remixes if someone has some better ideas for it.
This also works when printed at 50% scale, and is a nice even smaller alternative.
I'm not sure how much flavors will get trapped in the filament, I assume a decent amount, though I've only tried it with one flavor so can't confirm.