The stereotypes will have you believe an admin or engineer's job is strictly bound behind a monitor where the only tools one would need include a mouse, keyboard and a monitor. This really couldn't be further from the truth, and the amount of crap an admin needs for his various tasks could throw your back out.
The following is an organized series of sub-lists that make-up a larger Admin's Toolkit. These should help you with just about anything you could encounter out in the wild.
The backbone of your progress, your computing platforms will be heavily relied on.
Hardware and OSes change over time, but the essence of the workhorse laptop hasn't changed much since the mid 90's. This device must be reliable, just fast enough to be nimble and robust enough to be versatile in tasks. The idea is an entry to mid-level desktop replacement computer for basic to moderate computing.
Today, your main device should be able to run at least two virtual machines with something like Hyper-V Server, whilst also handling its typical duties.
Personally, I hate smartphones. But in reality, you need: a GPS, a calculator, a calendar, a flashlight, a portable web browser, a password keeper, a camera, a.. well, phone. Having all these separate is just impractical when a good smartphone can do it all together.
Best advice: avoid the flashy crap, don't overspend. You can get amazing smartphones for about $200 on Amazon.
Adapters & Cables:
Humans are adaptable creatures, and we've made our technology in the same guise. You'll need a stock of particular adapters and cables to hook shit together.
This one goes first only because of how much pain DisplayPort has caused me and countless other professionals. This may be irrelevant in years to come, but as it stands manufacturers of your typical production machines (Dell & HPE) have given up the tried and true HDMI standard for the cheaper and featureless DisplayPort. This is a huge problem because no display on the planet uses DisplayPort, and it sucks anyways.
Mostly used for testing or for expanding a system's connectivity in a pinch. I've left mine in a "borrowed" state at a job site more than once whilst the client scrambled for their own adapter.
Same as above, and also as above I've left mine hanging around at a job site, "borrowed," for the same reasons.
You'd be amazed how often a good cable will save your ass.
Helps in certain binds and doesn't take up much space.
Cisco, Dell, FortiGate, HPE and many others rely on the classic serial console port for deep troubleshooting or recovery.
A roll-over cable isn't much good without a serial port.
Whatever your devices use, especially your smartphone, pack double if you can.
Engineers are especially bound to needing a set of handtools. Thankfully, most jobs can be tackled with just a few.
A good, small-scale screwdriver will often have storage for bits in the grip, and can be equipped with philips, flat & torx right out of the box. Invaluable.
I fix, with my pliers.
Not used as often, but really great to have when you need them. Preferably a pair strong enough to cut light metals.
Preferably a Swiss-Army for a pocketknife, at least in the sense: something with accessories attached. Self explanatory, you WILL use it.
These items may not always be needed but you'd be sure happy to have 'em.
Save your wrists - a trackpad will just hold you back and hurt you in the long run. If you can find an ergonomic edition of your preferred pointer device, go for that.
USB hard drives and flash drives are just a requirement to work these days. Stuff has just gotten so much bigger and requires a lot more room. Sometimes, sneakernet is still viable.
They're small, effective and deliver unto you peace in a whirring server-room.
Not always needed since your smartphone should be able to tether, but very nice to have on its own. These devices tend to last a long time, perform well and also often serve as a battery backup for USB devices.
It's just professional and people like that.
Whatever you can scratch out ideas on.
Save your back - the mobile office can include car storage for those bulkier or lesser-used items.
Ethernet, adapters, power, whatever makes sense to your typical workload and what you work with.
When you gotta bring your own network, come prepared.
Do up some Ethernet cables of your own or make repairs to existing infrastructure.
They've already got your contact info on them, and you can add extra notes (steps, passwords, hours, whatever) on the back. Handy for handling client communication.
OK, this isn't directly related to work, but it's related to you and what you need in order to perform said work.
Keep comfort items in your car stock like your favorite snacks (at least the ones that don't go bad quickly) and if you're a coffee drinker, keep some brew handy. K-Cups are disagreeable environmentally, but a lot of offices dispose of these in a good way and having your own supply means you're never short of that addicting as Hell pumpkin spice (even in April).
It's strongly recommended to just acquire and maintain an extra set of "I wasn't expecting to stay" clothes for those times when you just weren't expecting to stay. Jobs run long, accidents and random-ass problems happen. Instead of sitting in your underwear at the laundromat or driving home at an absurd hour, keep a spare outfit so you can get a meal, hit the hay at a hotel and strike again in the morning.
Toiletries are self explanatory and unique to everyone. Just be efficient and make a kit as if you were going on vacation, that way you're not caught off guard and left feeling crumby.