5-gallon carboys

Our local water service are changing their 5-gallon carboys to a more favorable recyclable plastic. We ended up with about 400 carboys after discarding the un-useable ones. now I've run out of ideas for their use.
Some were used as:
reservoirs under self-watering beds
winter starting of seeds and then the tops used as cloches once plants are in the garden
starting containers for cuttings
Chicken feeders/water jugs
Ollas
Wicking reservoirs using nylon rope
Hanging self-watering pots
Any other creative ideas?

Thermal mass

Terrarium

Cut out the front part of the jug large enough for catch all for keys, small toys, etc.

Fill with water and use as exercise weights.

Birdhouse

Then there is a youtube for that:

housing for smallish animals (cats, chickens)
cut off top and bottom, stack them together and use them to organize firewood
keyhole compost deposit (like an olla but for compost/worms, cut off bottom)

Wow, great score!
The transparency is  the key quality they have over a 5 gallon bucket.
Because of that,  the cloche/ wintersow uses would be what u focused on.

If they happen to be the style with a side handle, then cutting off the top or bottom (save for another use) will yield either a giant scoop or bin with a handle good for something lightweight/loose, OR a giant funnel with a side handle to hold it steady or mount to *something* could be good for filling birdfeeders, or feed bowls from a sack of grain, or transferring liquids from/to large containers or attach a length of lay-flat hose to the mouth to send it someplace else!
Of course you can make a funnel without a handle if that's the carboy style you have, and you can also make it the full-size, so you could pour a 5 gallon pail into it all at once rather than at the pace of the neck and holding that pail the whole time...

You could make various styles of trap (mice, insects, fish, crustaceans...) and be able to visually inspect without opening or getting close.

Something to do with sound, a listening funnel? a resonating chamber for a loudspeaker?

A weather resistant enclosure. Maybe a safe spot for personal electronics while in the garden? A hood over a spotlight, or a game/trail camera to keep rain/snow off for better operation (possibly painted to reduce unwanted glare, either emitted or received). Maybe a storage for spare clothes/rain gear, tools, flashlight, or a first-aid kit in a remote area of your property.

A shop-vacuum funnel to collect more dust from a saw or sander in the woodshop. Possibly a cyclone separator for the same?

Hamster or gerbil habitat/cage. Observation beehive? A birdfeeder

"windows" or skylights for dark outbuildings

carnival games, like a ring-toss (onto the necks) or toss into the carboys, with varied sizes of openings by cutting neck at different heights. Race to fill a carboy some distance away, using other carboys, or something like plastic soda bottles or Solo cups. Guess how many (insert things here) are in the carboy.

windmill or waterwheel vanes/scoops. Scoop for a Wirtz pump.

I forgot cone-bottom compost tea brewer, with air-lift pumps using an aquarium air pump...

Are these the old-school #7 polycarbonate water jugs?

I have scored at least 50 of these from my local recycling depot. The staff fish them out of the recycling bin, and set them aside for me instead of tossing them into the landfill bin. Note: be nice to the peeps doing the dirty work -- it pays!

I think I posted this previously -- but I cut off the tops with a fine tooth wood saw, smooth the edges with a rasp, and use them for damn near everything. Because they are almost indestructible, they should be used long and hard IMO.

Primary uses for me:
- Clear storage bins for my sheds and shop, so I can see what's in there, up on the shelf, and there's a handy grab handle too
- Scoops for moving water (or snow, or compost, or biochar, or stinky anaerobic compost tea)

As suggested, cutting off top and bottom makes a fine sleeve that acts as a mini-greenhouse in the garden. I have maybe 10 that were damaged but work beautifully for that.

I use old carboys to make cider: apple, pear, grape and peach! Could call them sparkling wines? Once fermented carboys keep still juices (wines?) for at least a year or two until it's time to add a bit of sugar and syphon it into bottles. Then a few weeks more until there's enough bubbles! Bottled ciders then last years! I used to brew beer and ales in them too! Below are two 5 gallon batches of peach juice becoming peach cider.

Maybe some grow tubs like these cow mineral tubs shown here? Drill a hole in the side about 5" up from the bottom for water drainage.

A glass carboy at the local winemaking supply places sells for about $65 CAD.  I'd use them for wine as a primary use.  Some of the stores have gone to strictly plastic, so for me, while heavier, glass is becoming a premium...I want less plastic in my life, not more.

Cut off top and bottom and sink one in the soil for planting mint or something aggressive by rhizome like mint.

A general clarification -- I think the OP was talking about plastic 5 gal / 20 L water jugs. Everyone is moving from the old polycarbonate ones to the new, recyclable PET ones. As such a lot of the old ones are being tossed in the landfill.

Glass carboys in good condition always have value (including resale for cash) for people who make beer, wine, and cider. And perhaps some of the fermented health drinks as well.