I have a "here we go crate" with:
Light bulbs, tools, lockboxes, zip ties, gloves, tape measure, flash light, trash bags, gaffer's tape, pruning shears, varies batteries, common white outlet/light switch faceplates and screws, dog treats, and of course Excedrin for the headaches 😂 

1. Rechargeable flashlight

2. Toilet paper

3. Extra charged battery for my phone in case it gets low

4. Measuring tape

5. Business cards

I echo essentially what Thomas J. Nelson, REALTOR ® CRS,ABR,PSA,RCS-D, CFSP lists minus the Excederin.  When a new surprise need arrives, it gets added to the plastic tub, which also has paper towels, toilet paper, and some cleaning supplies. 

tape measure and flashlight

A good flashlight.

It is also a good idea to have some old shoes when you need to walk in wet grass.

John Palmer my phone charger and small tool box.

Business cards! 

Hi John- as a stager, I always had tub that included paper towels, Windex, scissors, tape, and plastic trash bags. 

There's a whole 'Realtor® box' but, the #1 thing that I use the most out of it is my tape measure. It was my flashlight until we got those on our phones!

Silicone spray . . . 
that is just one of a million things in my truck . . .

To assist with driving:   

A Garmin GPS, an Escort Radar Detector, and a AAA Card.

 And for good measure,  a defensive weapon and a CWL to ensure compliance.

I'm in SoCal, the temperature drop can be drastic, so something warm to cover shoulders, nice heels if needed that day. Basic staff: a screwdriver, extra batteries, few lockboxes, a bag of keys and remotes, a tape ~ few things to mention:) 

Duct tape, misc. tools, band aides and hand cleaner for starters John Palmer 

Most agents in my market don't own a car, nor do I. Subway, Bus, Uber, and walking is how we get around. On rare occasions we may hire a car service for the day. To adjust for your question, I have a backpack. I generally carry extra charging cords and battery charger for clients phone. On rainy days I pack an extra umbrella, hot days - water for clients and for me, etc. I do keep a measuring tape, business cards, etc in my backpack. If it's a high end client- I have a briefcase rather than a backpack. 

I must have been a fly by the seat of my pants agent, I never had anything specific. The most common thing I needed was a pen. So I still have one of those.

some tools, pliers, hammer, screwdrivers, measuring tape, flashlight, phone charger and water. And I have been known to run out of water.  

It seems like everything, including TP, except the kitchen sink John Palmer 😁

GPS

WD40, zip-ties, hammer , screw drivers, pliers & lockboxes 

Notebook and business cards

A phone.

Just like Thomas, I had any supply that I possibly may need to clean or repair, plus toilet paper.