Well, it's spring again finally and all those little things I didn't get last year, I'm determined to get for this year, so here are some recommendations.

Arm Warmers

Yes these don't really wear out, but I keep losing them, so there are quite a few to choose according to Cycling Weekly from the Carney Reflective Sleeves were originally a Kickstarter project, but you can get them for GBP 29 shipped anywhere worldwide. They are a nice safety item and with biking, I'm always big on that.
Having gotten them, they do run a bit small, so if you are thinking medium, then get a large. They are beautiful thought and reflective.

Leg and Knee Warmers

Most of my knee warmers have warn out, the grippy thingies just seem to fail after a while.
Bicycling recommends the Assos legWarmer_ev07 for super cold days as they are fleece lined which are an incredible $89 but well worth it mainly because they are longer and don't have the elasticity problems of traditional warmers. And yes they do have a weird name, like someone forgot how to capitalize or use spaces. They are$89 at Competitive Cyclist and their sizing is a little strange too, but 0=small, 1=medium and II=large.
Cycling Weekly recommends the The Assos kneeWarmer_evo7 is more reasonable and also doesn't use grippers, but they go under your lycra. A nicer design and in trying them, just make sure that you obey the labels and put them on with the seam out. They have a huge overlap with your bib shorts, so that is way better than other designs. Lightweight so good for autumn or spring days.
The super light Castelli NanoFlex+ is another recommended choice for milder temperatures. The nice thing about them is that they are waterproof (although I wonder how much that really helps in the big rain).
If you don't need something that warm then the $19 Specialized Therminol knee warmers is a good choice but these do use grippers. Helmets Well first of all if you have an old helmet, it might work fine, if you just get a new set of pads for it, the Giro Aeon replacement pads are just$6 at Excel Sports. They might now exactly fit your helmet, but certainly better than replacing a \$200 helmet just because the pads wear out. But remember these other ones do not have MIPS are other improvements.
For helmets themselves Cycling Weekly, Outdoor Gear LabsBicycling, Road.cc and Bike Radar the latest trend is going more Aero (but this is warmer) and also MIPS which is supposed to reduce rotational impact. I don't know if it works, but why not get the most protection you can afford. Now if you haven't had a helmet before going to a store to get fit is a good idea. The drawback of MIPS is that it weights more and aero means you don't get all the cooling you need at low speeds.
For me, I find that I can fit most medium helmets from Giro and love the Aeon which is super light at 225 grams, so here are some updates to try...
Giro Synthe MIPS. This is a relatively light and yet aerodynamic helmet with the full MIPS package. It comes on top of nearly all the reviews. (but beware if you have issues with the new owner of Giro, Bell,... although they are about to get sold into another entity). If you really just want the lightest, the Giro Aeon is the one to try. This is very much the benchmark of comfort because of its fit system, aerodynamics and Multi impact protection system.
But if you are looking for other choices, there are other more specialty brands around mainly from Europe and the big contender is the Kask listed below...
Kask Protone. This is a very light 215 gram helmet for small, but 265 grams for a large but it doesn't have the MIPS protection (this is a proprietary pad that reduces rotational impact). But everyone loves the feel and it is used on the Pro circuit plus has a cool European look. But Outdoor Gear Labs did not like it's adjustability whereas fit didn't seem an issue for Cycling Hacks and Cycling Weekly was that ventilation wasn't as good on hot days, that's the price of aerodynamics. They do like the full wrap around plastic that protects the foam, but the decals will fall off. I actually found it does run a bit small, so I needed a large. It is a beautiful helmet though.
Lazer Z-1 MIPS. This heavier at 284 grams in medium but does have MIPS and with Outdoor Gear Labs scores nearly as well as the Giro Synthe although it is not an aerodynamic helmet, I'm not sure that matters for ordinary riders.
Then there are some more exotic choices...
MET Trenta 3K Carbon. This doesn't have MIPS but does have a lighter foam system and should be a true but isn't available anymore it seems from Wiggles which is too bad
Specialized Prevail II. Weighing at 200 grams it also does not have MIPS but is very light. It is an older design that has been wearing well 🙂

If you have a heavy one, it's an easy way to drop 100 grams. In this case, in looking at Outdoor Gear Labs and other sources, they like the Fabric because it is very light. The Fabric is from the UK and comes in three shapes, flat, scoop and shallow. The Ultimate is their carbon on carbon fiber and very light. About 165 grams light. The flat is best for those who are in good shape and like having a bunch of different positions.
The other one to try is the Fizik Arione R1. This is a carbon fiber base at 180 grams. Fizik has a cool system where they rate their saddle based on how flexible you are (Snake to Bull) and then on how much power you put out to determine if you need a 130mm wide or a 145mm wide saddle. I have Ariones on two bikes and they are ok, although because they are scooped, the fit has to be pretty much right on. One of my bikes has run out of rear adjust, so it is just a little short for me.

Through Axle for Disk Brake Bikes

If you are running a disk brake set on a road bike, then you likely are using conventional skewers. the issue is that you can possible have the skewers fall out because there is more torque. The solution is to add some weight and get the DT Swiss RWS skewer, these add a thru axle (which does not allow the release of a wheel accidentally. They do add 100 grams more weight though.
The main pain is figuring out how long a thru axle you need and what kind of diameter, normally 9mm for road bikes. Traditionally it is 110mm in the front and then 125mm in the back, but disc brakes need more.
Installing these things is pretty simple, just insert the axle, put on the wheel and then hold the nut end while rotating the handle end. When you are done, there is a ratchet that holds everything in place. It can't come out easily.

PowerTap P1 Cleat Cover

These are special cleats for the PowerTap P1 power meter, the issue is that they are Look compatible but not quite the same, so you need to order a special cleat cover. The Exustar E-CK1B should do the trick.