If you haven't already gotten a Steam Deck - I highly recommend it! They're in-stock (unlike the waiting game we had to play before) and there are knowledgeable communities and sites out there to help you get started. The case that came with it is already a great one, so I haven't even bothered looking for anything else.
Here's my two cents regarding some essential accessories for the Steam Deck!
JSAUX - dead simple dock - it's just a stand, but it does the trick. Cheap enough to have multiple ones (I have two!)
Some - not all - links may be affiliate links, but most will be for products that I have used or currently own.
Dock
They nailed it with the form factor, but when you're going to be on a desk or table top for awhile, it's best to have a dock. We started out with simple "dumb" ones (which are still great for quick setups) and then got full-fledged docking stations from multiple manufacturers. You'll see tons of recommendations on Amazon or other sites, but here are my top two picks:
The Offical Steam Deck Dock - comes with a power adapter and probably the best support since it's direct from Steam. I think the only downside is that tethered cable. I wish they had made that a standard, high-quality USB-C cable and let people know the specs required in case they need a proper replacement that can support video, PD, etc. You'll get 3 USB-A 3.1 ports, Gigabit Ethernet, DisplayPort 1.4, HDMI 2.0, Multi-monitor support, and FreeSync support. Bonus - there's updateable firmware so keep an eye out for updates!
Chargers
These are my two go-to chargers for moving around the house or travel (road trips!). One is smaller and compact with a "legacy" USB-A type port, while the 100w is a full-blown USB-C only charger that I can also use to power a non-gaming laptop in a pinch. I've used both for more than 6 months at this point, and they have been rock-solid.
UGREEN 100w USB-C GaN charger w/folding plug - If you need a compact, dense charger with up to 100w of power for your laptop, tablet, phone, and Steam Deck, this is it! Yes, you will notice a "disconnect/reconnect" if you connect a second device while the first is charging. This is so it can allocate power accordingly (see diagram above), but it's been a great addition to my travel bag.
UGREEN 65w USB-C GaN charger w/folding plug - Though obviously not as powerful as the 100w version, this is a great compact charger. You get the two USB-C ports and a USB-A port because we're still not rid of those darn things yet! This is a good, small, flexible charger to keep with you wherever you go. NOTE - the only "con" is that when powering a USB-C monitor with PD to the Steam Deck, the Deck reported a "low powered charger". So apparently this can't pass enough power through another device. Using it directly was great, though.
Cables and Adapters
Cellurize Right Angle USB C adapters - If you've already got plenty of nice USB-C cables that can pass video, treat your Steam Deck right by using 90 degree adapters to reduce any possible strain on the charging port and keep the cables looking nice and tidy. A 4-pack of these is cheap and all you'll need for home and travel.
Cable Matters - Slim USB-C cables - for charging only (not for docking/video), I love these. Paired with my favorite chargers (see above), they make for a nice portable gaming solution. Can't beat the price for a pair of cables, either!
Cable Matters - 6' 4k USB-C, PD cable - This it the "nice" cable you don't want to lose! It will charge and provide video to your dock or USB-C monitor (up to 4k/60hz which is more than the Steam Deck can really handle, gaming-wise)
Sabrent USB-C 6-in-1 hub with HDMI - I didn't want to put this in the "Dock" category, but that's basically what it is. I wanted a small, portable solution for more ports and HDMI if I needed to travel, and this was it! I've tried it on a 4k and QHD (2560x1440) monitor and it's worked great. Especially when you need to attach a keyboard to edit some things in desktop mode. My only "con" is that it got quite warm to the touch, but it didn't seem to affect any functionality. The small size makes it perfect to tuck into your Steam Deck case's little "cubby".