Reddit mentions of Eight (8) 625ZZ 5mm x 16mm x 5mm Shielded Deep Groove Precision Ball Bearings

It honestly depends on your budget, how much are you willing to spend?

Here is a great list of prep and upgrades you can do to bring success to your prints:

-Get Vibration Dampners: I can not express how much joy these things have brought many users. The make your machine quiet and greatly decrease ringing in your prints since they ABSORB the vibration coming from your stepper motors. You really only need Two of these for the X and Y axis however you can get another two for the Z-axis drives if you plan on doing a lot of z-hopping.

-BUY FILAMENT IN ADVANCE: get some PLA/ABS/PETG or whatever you want to try printing with. The filament you get with the printer for the most part is garbage HOWEVER I recommend dialing in those settings first then throwing on some nice filament, it'll feel soooo nice. Some commonly accepted/appreciated brands are: eSun PLA+, Hatchbox, Solutech and more

-Get a PEI sheet: Seems like you mentioned this in your post but PEI sheets eliminate the use of gluesticks/tape/hairspray and provide excelent adhesion while making it easy to remove the part after it's cool. This things like to be hot though so increase your bed temp a little at first

-Buy New Fans: The stock hotend/power unit fans are Loud, Noisy and Inefficient. I recommend getting 1 new conrol box fan, 1 new hot end fan and 1 new part blower fan.
You will need 1 40mm fan, 1 50mm fan and a Blower fan or another 40/50mm fan depending on if you print a new hot end mount like a fang.

-Tighten every screw on the printer: You would be surprised how many screws initially are lose either from prior testing or such. Also make sure to test your belt tensions on your printer whne you get it. You want everything tight BUT NOT TOO TIGHT. Give it a snug fit then let it be. Also make sure your belts are 'pluckable' like a guitar string but not tought

-Get some bearings: You may want to get a few ball bearings for when you print out a new filament holder, the stock filament holder is terrible

-Buy some fasteners: M3 bolts and nuts are commonly used on the CR-10S having spares or additional fasteners for adding on parts/printed components to your printer is awesome to have on hand

-Replace the Hotend: This can get a bit frustrating but it can be worth it. If you want you can replace the hotend with something like a volcano or an all metal hot end to print at higher temperatures. With this you can also add on autoleveling if you have the correct mount printed

-Octoprint: I am not very familiar with octoprint since I don't use it myself however I know it can run on a CR-10S. If you want look into how to install octoprint on stock firmware, I know it's pretty easy to use a raspberrypi to remote into your printer as well using octoprint.

-Squash Feet: Replacing your stock machine 'feet' with squash balls or squishy gold balls can also help absorb vibrations from your machine to your table or where it is located. This helps a bit with part quality but it's mostly to reduce the noise carried through your furniture.

-Solder on connectors: If you want you can get some pin connectors to make your fans/electronics easy to switch out depending on your material you are printing, maybe you need a strong part fan or maybe you need a weaker one. It is easy to swap out if it's using a 4pin connector.

-Make an Enclosure: What could be nicer for your printer than a stable environment. Making an enclosure for your printer can help if you live in a rough climate area or if you want to reduce the noise as well.

-Get some Handtools: I'm not sure if you have any or not but some great tools to have are a screwdriver, hexwrenches and a soldering iron depending on how much work you want to be doing on your printer.

For convenience I've added some of the parts mentioned via an Amazon link for prices. I'm not saying buy these things exactly they are just a starting point for reference:

Nima Vibration Dampners:https://www.amazon.com/Stepper-Rubber-Vibration-Dampers-Printer/dp/B073FRZTDX/

40mm Fan (Noctua Brand): https://www.amazon.com/Noctua-NF-A4x20-FLX-premium-quality-quiet/dp/B072JK9GX6/

Blower Fan (Part Fans):https://www.amazon.com/SoundOriginal-Humidifier-Aromatherapy-Appliances-Replacement/dp/B071WMHNG5/

PEI Sheet/Printer Surface:https://www.amazon.com/CCTREE-Printer-Surface-Creality-300x300mm/dp/B07543KHCT/

Squash Feet: https://www.amazon.com/PrideSports-Practice-Balls-Count-Yellow/dp/B00466W9X0/

M3 Bolts/Nuts:https://www.amazon.com/280Pcs-Grade12-9-Socket-Assortment-Storage/dp/B0742DDLQ1/

Raspberry Pi 3 for Octoprint: https://www.amazon.com/Raspberry-Pi-RASPBERRYPI3-MODB-1GB-Model-Motherboard/dp/B01CD5VC92/

Ball Bearings: https://www.amazon.com/625ZZ-Shielded-Groove-Precision-Bearings/dp/B01LWMT95S/

Hope this helps, have fun tinkering!