I wear one of those bright yellow jackets, as well as have extra reflective "stuff"-leg bands, gloves, & on my pannier [see pic]. If its too hot for a jacket, I'll usually be wearing an "obnoxiously colored" cycling jersey. When I commute on one of my bikes without a rack (no pannier) I have extra reflective stuff on my bacpack [see pic].

I carry extra clothes in my pack/pannier (eg raingear+gloves+hat), and my tools, pump, patch kit, spare tube and tire levers.

Work clothes ("scrubs"), I also have extra clean sets at work. Small towel, hairbrush (or kept at work).

I try to keep a stock of extra snacks handy (cliff bars/snickers etc) which I'll switch out (eat) to keep "fresh"-handy for nasty days when I'm tired riding into a headwind.

Cell phone, wallet, work ID.

Work stuff (Technical manuals, paperwork etc)-I try to have duplicate texts at work, so I don't have to lug them around when I'm on call and need to review procedures.

Lunch-a few sandwiches, snacks, fruit.