How to maintain the bicycle after a commute, when not home? When I commute, which is a 20 minutes bicycle trip, I'm able to park the bicycle indoors at a facility, where I can rinse water off of it. See my other question regarding whether or not this is a good idea: Should you hose down your bicycle after having used it on salty roads?

However, I'm given the impression I should not only rinse off of it, but also lubricate the drivetrain after doing so. To do this twice a day is a stretch, but if I'm going to consider doing that on top of rinsing salt water off of it. What should I bring?

This artificial limitation of "one item" is not meant as a joke, I'm sorry, but I simply don't have the energy or time to do full maintenance of the drivetrain after such a short commute. But a simple spray or oil is perhaps possible to add to my routine.

I'm sure others find themselves in a somewhat similar situation, perhaps not with a facility to rinse off the bicycle, but nevertheless perhaps could bring one small spray or similar to maintain the bicycle with after every commute.

So the routine in my case is: commute on roads with salt and debris, rinse the bicycle with cold freshwater at low to medium pressure. Then, what?

What should I bring, how should I use it (chain, entire drivetrain?), and why?

WD-40? Oil? Just a rag? Some other spray?
Do nothing.

Yes, nothing. Unless you are biking wearing a hazmat suit through some corrosive volcanic crater where it rains acid.

Rinse it off, at home when it's convenient, or it is especially dirty. Give a more thorough cleaning once a year in the spring. Lube the chain occasionally when it begins to make some noise. Other than that, you really don't need to screw around hosing your bike off so much. Just ride, it'll be fine.