This is what our under-sink cupboard looked like when we first moved in - there's an additional (removable) panel on the floor for the original drainage, plus a hole at the side for our dishwasher pipes.
We wanted to keep the panel accessible in case of any sudden leakages (touchwood!) so we decided not to add in any bulky storage/organisation items here.
And this is what it looks like after it's been filled with all our stuff (accumulated after two years of living here LOL).
We lined the bottom of the cabinet with a protective liner from IKEA to prevent any drips/spills from going into the carpentry, because that's very likely to happen when you have a bunch of cleaning solutions stored here :')
On the other side, we have a couple of stackable plastic drawers to hold small items! Chose the transparent ones so that we can clearly see what's inside, even though they don't look as sleek (but they're hidden inside the cupboard so nobody's going to see them anyway)
The top drawer contains mostly refills or backups of products we use for cleaning - descaling powder (for coffee machines, water dispensers, etc), decloggers for the sink and extra drain netting, dishwasher cleaners and sample dehumidifier packs.
The "currently in use" versions of these are stored outside the drawer for easier access and also to avoid us opening multiple bottles of the same thing (has happened before LOL)
The second drawer contains 'less frequently' used backups and items, such as our drain stopper (we use it when we travel!), extra trash bags we got as a free gift, drain covers, extra kitchen hooks, etc.
Essentially it's kind of like... a junk drawer, but for, uh, useful junk HAHA.
And on top of the drawer unit, we have strategically positioned our Slurpy Bear dehumidifer box for *maximum absorption* (just kidding - it was just the most convenient place to put this LOL) as well as our dishwasher tablets!
We keep the tablets here because we don't use the dishwasher every day and don't want it to take up precious counter space.
And on the other side of the cabinet we basically have a bunch of 'big' cleaning items and refills like dishwashing liquid refills, floor cleaner refills, etc etc.
We have a lot of these because we tend to buy them during sales (and also we don't really change the brand of our cleaning products once we find one that we like).
This is also where we keep our 'heavy duty' expensive cleaners (e.g. the Krafter magic sheet and the mamaforest bubble cleaner) - which we use for major kitchen crises like burning our pots or accidentally staining the kitchen countertop with something that normal cleaners cannot remove. Sounds strange but it... happens.
AND FINALLY, behind all these things, we have a secret tucked away section where we keep our pest control Baygon (haven't used it much since we moved in, and I hope we never have to use it LOL) as well as some random DIY tools (e.g. scrapers, goo-gone for tough adhesive stains).
These are the things we don't use frequently at all, but we'd want to keep them accessible because the situations in which they need to be used are typically emergencies LOL. Hence, they're tucked at the back of this cupboard and not in the depths of the storeroom.
And that's all for our under the sink storage! Hope this helps you in planning for your own cabinet, if you still have no idea what to do with it :')