I use a fanny pack (or lumbar pack) for carrying a thick cable lock and other items when I need them. Something like this.
I have a small under seat bag to carry tools and flat repair items. These always stay on the bike. But I use the fanny pack if I need to carry items when needed. Keeps the weight low instead of on your shoulders, and doesn’t get the back sweaty.
As others have said, put the weight on your bike frame, not on your back. If you don’t want to mount a rear rack, there are so many packs that mount on your handlebars, the crossbar, behind your seat. Don’t get the biggest one you find - get the smallest one into which you can fit what you need. I fit a tube, some tools, my phone, and other small stuff in a tiny wedge under my seat. They would just be rattling around in anything bigger.
You can mount a pump/u-lock/water bottles on your frame.
Wimp! 
Here near Chicago, my sister and I make a point of biking at least once every month. This year it was no problem. So long as the streets are dry and clear of ice, clothing is effective - especially if the sun is out and it is not too terribly windy. Under 20F - it can be easy to convince ourselves to stay home!
This. Weight distribution matters. It’ll be more comfortable if it’s on the bike instead of you for both weight & cooling reasons.
IF you still go with a backpack & want reflectivity think of your riding position. If you’re leaning forward on road bike style bars that reflectivity up high will be great for a police helicopter searching for someone from above but not so much for the car behind you.
Lights are better. There are many options available, at all price points but don’t go to the store & hold one up (especially with a single LED) at arms length & say it’s really bright & buy it. Turn it on & walk as far down the aisle as the store will permit. Is it still really bright from 20’ away? 50’? How bright is that single LED if it’s not perfectly angled towards you? A driver needs enough distance to see the light, understand what it is & then take some action, whether that’s slow down or go into the next lane. Some of those that are blinding at 2’ are BS on the road. I’m a fan of Guardian Angel lights; bright a f.
When I recently had my bike tuned up, they suggested “daytime running lights.” My experience with lights is they are just one more thing to maintain. I’m a kinda big guy and ALWAYS wear a dayglow yellow vest when biking (and never bike at night.) If someone is gonna miss THAT, I’m dubious how much more help any amount of lighting would provide. (Vest cost <$10 >10 years ago, and has required ZERO batteries, maintenance, etc! ;))
I mean “real” riding. I commute year round and I currently have studded tires on my commuter bike but I think it’s safe to swap them (we do get snow in March and April but it is usually short-lived). This afternoon I’m going out for a gravel ride - looking forward to it!
I have a bright pair of lights (headlight and red taillight) that mount on my helmet. If I’m going for a road ride, I put them on. When I’m driving and there are cyclists out on the highway, I notice the flashing lights first. I think they work.
BTW, I wear bright clothing. I’m also someone that has been hit twice while on two wheels and my wife was ran over by a car while she was crossing the street (she was seriously injured while I came out unscathed both times…same can’t be said for my 2-wheelers). I take ever precaution to be seen that I can.
Cool. Was pretty much yanking yer chain re: being a wimp. Good on you for the commute. Mine is short and ugly enough that I prefer to drive and bike when I get home.
Lights are probably a good idea - especially as they’ve gotten so much cheaper. Almost disposable. But the vast majority of my biking is on shared rec trails (rails to trails) where the only cars encountered are in street crossings - which we ALWAYS take carefully. As we often say, we don’t want our tombstones to say, “He/she was right!” 
I’m just thinking back 5+ decades of having batteries leak in neglected bike lights! 
I don’t put the lights on for those types of rides: We have some fantastic rails-to-trails around here and I don’t see a need for them there. And yes, leaking batteries suck! The pair I have have integrated lithium batteries - rechargeable. Small and bright.
Isn’t that much less of a problem with modern batteries and light bulbs?
These headlights may be a good idea for riding in traffic, but they are super annoying and unnecessary on the bike trail, especially when in “strobe” mode. If you are on the bike path there is no need to blind/cause seizures to other trail users - there is a reason no one is waving to you. We can see your old man chartreuse jersey just fine! (not directed at @Dinsdale ).
Plus, they seem most common on e-bikes - which I DETEST!
Thrift stores usually do a good job of keeping personal carry bags to their own area so it’s easy to visually scan the whole stock quickly. Soft bags like backpacks, messenger, laptop and purses seem to top out around $10, unless exceptional. But you could easily see dozens (100?) of different backpacks at a standard Salvation Army or Goodwill.
To be clear although I have done loaded touring, the VAST majority of my rides are short commutes. I biked to work fairly often – one pannier had a change of clothes, the other had tools and lunch (no need for lock as I could bring my bike into office)
Even when mostly empty I usually have 2 panniers on my rack, though for some rides it is just my seat bag.
If I need a lock (not often) it is either in a pannier or perhaps bungeed to my rack (I have a U lock)
Brian
Was in Holmes County, Ohio last weekend. All the Amish teens were zooming around on those things.
Put the weight on the bike not your back
I’m thinking back to when my bike had a little generator that, with the flip of a lever, would tilt in a bit and engage with the back tire. The faster you rode the brighter your light was. As a kid I loved pedaling as fast as I could to make the light as bright as possible (and I liked the sound of the little whine that came from the generator).
And when you stopped at traffic lights your lights went dark. Aaah the good ole days - when things were often crap.
My front light is a contact-less version. There are magnets in the light which is near the rim. Eddy currents are generated in the rim which causes an opposing magnetic field which spins the generator. There is enough capacitance that the light stays on a while after stopping.
Brian
To be fair, it was a product of its time. Safety from others was not the point. Lighting my way while riding was (and I am not sure it was good at that). It was on me to avoid traffic and dangers…not the other way around.
The shit we did and got away with in the 60s/70s (and probably earlier) would horrify most parents today. It’s a wonder so many of us survived.
Probably a topic for another thread, but I’m not so sure today’s kids are any safer than we were, in spite of all the additional layers of safety and modern parental angst. Were kids really getting maimed at higher rates back then?
Heh, those were old-timey when I was a kid in the 80s and everyone acnowledged that they suck. But I also remember the whine and riding really fast to see how bright it would get: whispy ember was really moving!
My escooter has a headlight and, in addition to alerting others, it’s quite effective for lighting the way. The beam throws low and not too wide but still illuminates close enough for me to see the small wheels over cracks and around holes.