Lens hoods - agreed they should be used at all times. Also come free with purchase of any Sony lens.
Bulb blower - for sensor cleaning, this is approach #1, and usually handles 85% of all dust removal needs. Priceless.
DSLR sensor brush - for me, this is sensor cleaning #2 - gets another 10% of the stuff the bulb blower couldn't. Also priceless.
Wet swab sensor cleaning kit - cleaning #3 - for the ultimate stuff that won't come off via any other method - usually something you only need once a year or two, unless you frequent dusty, dirty places and change lenses a lot. I only use this when cleaning #1 and #2 fail to work.
Aftermarket strap - I hate the straps that come with cameras - nasty and uncomfortable especially for long lenses, so going with a nice wide neoprene strap to distribute the weight and remain comfortable even with much weight on it and sweating and hot. My Kata Reflex E strap has been aboard my last two cameras, and will continue to be on my next ones.
Microfiber towel - something I always keep handy in my bag - usually 2 or 3 of them - for cleaning lenses, camera bodies, battery ports, connection plates, etc. Good, fine, scratch-free fibers, static absorbtion properties, and no lint.
Wired remote relase cable - wireless remotes are cute and fun, but for me, I like having a power-free alternative that will work no matter what, powered by the camera itself, in case of any battery failures, signal issues, etc. There's something I'm just intrinsically more comfortable with for long exposure work, and a non-powered, wired remote is reliable and always works, and I feel much more comfortable with the ability to lock the shutter open and leave it that way for minutes at a time without thinking about dying batteries.
Tripod - gotta have one if you do long exposure work, which I do. Don't feel it necessary to go for 4-figure carbon fiber uber pods - aluminum works fine for me, I can handle the extra 1/2 pound of weight or so, and don't need any fancy heads since I only use it to set the camera up, point in the right direction, lock it down, and expose. A good $100-150 pod does just fine for me - free form legs, good 60" height, very compact and portable and fits in a bag or locker, reasonably light, well padded legs for carrying. Happy with my Slik Sprint Pro EZ for the past few years.
Filters - necessary? No, not for me. Fun? Sure. I get occasional use from a good circ polarizer, some ND filters, a UV/clear or two on certain lenses, and some specialty ones like a 9-stop ND which is fun for daytime long exposures.
Bags - lots of them, every size. Being a bit of a lens collector and having two bodies, I have to deal with an assembly of two cameras with about 20 lenses. So bags of varying sizes let me choose the bag for the occasion and the lenses needed. I have a small camcorder bag that accomodates my DSLR with attached superzoom and can squeeze one small prime too. I have a small shoulder bag that can accomodate DSLR and two lenses plus flash or mirrorless body alongside. I have a medium shoulder bag that can accomodate both cameras, plus up to around 4 lenses - or one camera, and up to 6 lenses. I have a two-lens holster bag that accomodates both of my long wildlife lenses side by side, allowing me to switch between the two lenses on the fly with my DSLR - the lenses point mount-up out of the bag, so I can insert the mounted lens into the empty holster, unlock from the body, move the body to the other lens, snap the camera body on, and slide it out of the other holster. And I have a large backpack that acts as transport, and accomodates the DSLR and 7 lenses - including the two big wildlife lenses mounted on body if needed.
Sony DSLR-A580 / Sony 18-250mm / Minolta 50mm F1.7 / Sigma 30mm F1.4 / Tamron 10-24mm / Tamron 150-600mm / Tamron 90mm F2.8 macro / Minolta 300mm F4 APO
Sony A6000 / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / 10-18mm F4 / 35mm F1.8 / 16mm F2.8 / via manual adapter, lots of Pentax K mount, Konica K/AR mount, and Leica M mount manual lenses