What do you do with your collections?

I usually don't have complete collections because after a while I forget about what I was doing, but at one point, I do remember having a very outdoorsy sim that was collecting minerals. I built them a shed near the garden. Along the top of the walls, I placed several shelves so that they basically made a circle that went around the entire room. I believe it was the simplest little shelf in Build/Buy (it was the one that doesn't have any embellishments on the ends, because I had several of them right up next to each other and I wanted them to look like one continuous piece of wood as much as possible). They were high enough that I could put the flower arranging table/other garden-related objects underneath them and have a functioning shed. I never used MOO or resized anything.

I play rotationally so it depends on the sim. Some of them keep certain things, like snow globes, others might keep other things to display or use. I use shelves and tables to display them. Some things won't fit on shelves but a lot of collectibles do.

I just display my household's collections in the basement. On tables, shelving, etc. Pictures, like experimental foods, are on the wall.

Yes, the shelves will at least hold the snow globes and dolls. Maybe the largest crystals need more space.

If you are fine with cc, there are good display objects over at modthesims (by TheJim007, based on those we had in TS3. I could not make a link, but you can search for "Display Cases & Pedestals from TS3".

You could assign a community lot to a collectors club, then if several households collect similar stuff, they could add their collections there.
Or, you could "donate" a full collection to a Museum, perhaps add a photo of the sim donating it, and then all can go there to admire the items. You would need to imagine the story/process, but I don't think you will need cheats to make it work.

If you have Cottage Living, perhaps collected items can be used for those friendly neighbor gifts.

@Larrikateven @RedShoe7 @simgirl1010 @Simmerville
Thank you for your wonderful suggestions!

Fish can be mounted and hung on the wall, so those are easy. Snow globes are definitely on the list of things my Sims will collect, and if they fit on standard shelves, that would be wonderful. The minerals are pretty, too, so even if they require special shelving, I'll be willing to see what will work for them.

My sims only put some favorite pieces on display, the rest I store in chests or basement/attic.

But now I wonder if crystals, geodes and maybe also metals could get integrated into the landscaping. Doesn't have to be a full rock garden made of crystals, just a piece here and another one there. Or maybe sort your exhibitions not strictly by collection, but by other criteria, like color. You'd still have your main collection in its dedicated corner, but also, for example, a selvadoradan idol with a green gem inside next to green crystals on an end table in the living room.

Akira Kibo in my San Myshuno game has a little bit of a hoarding problem collecting bug. Here's the main collecting area. His very patient wife and roommate are making this work in an apartment with 4 small children.
https://i.postimg.cc/R0gb9bQz/move-collectible-room.png
https://i.postimg.cc/L6P7fDRz/move-collectible-room-2.png

Their dining room wall is also papered with his poster collection. Almost complete.... almost....
https://i.postimg.cc/sgNcvsPB/scold-for-potty-refusal.png

yeah if i bother to collect collectibles i mostly display them somewhere in my sims home but I find it frustrating trying to find tables/shelves that work for that purpose so I often end up having to moo something up myself

stacking dining tables, using platforms and walls to make it look built in shelf is probably the way i do most actual collection rooms

The gems have an item in BB where you could place the cut version. I forget the word, I have never actually collected them but you send them out via the mailbox or if you’re lucky you’ll find one after thunder strikes the ground. You can place them inside it to be displayed.
Edit: I was confusing the gems with the element crystals, my bad.
https://sims-online.com/sims-4-elements-collection-guide/

I don't usually keep the collectibles my sims find. I use some to decorate their house, but most of them I sell off as there isn't always enough space to display an entire collection let alone several. I did keep the bottles one of my sims found on the beach in Tartosa and made a feature wall in his kitchen out of them, but otherwise it is just one or two items of one collection and such. I have an experimental food restaurant and decided that whoever is going to eat there will take pics of their food and then put one up on the restaurant walls. And I have another restaurant with aquariums that I would ideally like to stock with different types of fishes some day, but I don't fish often enough and when I do the fish usually ends up in a dish. :) The things I find in Selvadorada go in the Selvadora museum, well at least some of it, as I needed to sell some off to fund further expeditions.