Probably one of the least flashy accessories, but I bought a 10 foot long USB-C cable that lets me sit anywhere on the sofa and play without battery anxiety.
Jsaux mod case for travel/backpack, 25000mwh battery pack, USB hub for connecting to tv and Bluetooth dongle for Xbox controller. Considering 3d printing something for the rear buttons as I find them a little shallow/hard to press. And spring for a microSD for emulator games expansion.
It really, really doesn't need very much in the way of accessories, if you find this and are planning to get one, I'd recommend getting it first and figure out then what you want after using it a bit.
@Fubarberry for on the go i have a foldable bt keyboard with trackpad and, if i feel fancy, a little projector that can be powered by a powerbank. For at home i have a 13" screen with an arm that i can clamp onto the nightstand or on the workbench and a cheap wireless logitech m+k combo. If i ever feel the need to game docked instead of handheld i'd probably use my xbox controller or maybe the steam controller
I mostly play handheld, and I got a gaming pillow from Mechanism. It sits on my lap and holds the deck at the right height and angle. My hands used to fall asleep from squeezing the deck to hold it up, and this has solved it. I would play docked more often if I had a controller with rear buttons.
I use a jsaux modcase instead of the included carrying case, it provides enough protection and fits better into my backpack for work. I've got a 3rd party ivoler dock I use for my TV, and 4 stadia controllers for docked play. Finally I have a walmart onn brand BT tablet keyboard and a trackball mouse that I can use while it's docked.
Dock, usb hub, usb SSD, wireless controller and a 65in tv.
One almost mandatory software accessory for me is KDE Connect. Allows me to use my phone as a mouse and keyboard in desktop mode, as well as transfer files between devices easily. Honestly this is a must have.
The Deck is a home console for me mainly, although i do use it while travelling sometimes which feels like an added bonus.
It is the most capable console/computer i have ever owned.
Long live valve. Im scared of what will happen to our beloved Steam once GabeN passes the torch though.
small bag of usb adapters, for charging friends' phones or connecting a flash drive, etc
dualshock 4 controller, modded with back buttons, a usb-c port, and a phone holder
slim mouse and folding keyboard
I used to carry a battery pack but its heavy and rarely used, so now I just carry a second usb-c cable and a c-to-c extender and hopefully i'll be able to reach an outlet.
I use this dock by Skull & Co. because I needed a second dock. It's half the price of the official Steam Deck dock and it has nearly all the same ports. It's always functioned flawlessly.
I have purchased docks, cases, and thumb grips from them and they've all been great.
Only things I use are, if docked, are a 3rd party wired switch "pro" controller (both because of the controller itself and because of the headphone jack on it), and an as needed my Bluetooth trackball mouse and the wired keyboard I use for my desktop.
The only other things I really have for it for accessories would be the case it came with (if you count that) and a USB C to USB male connector I got before I had a dock (for plugging in a thumb drive when undocked to more discretely move downloaded files or don't have internet access because I'm in a vehicle (mobile hotspot absolutely sucks)).