I'm not very inventive, so you can take these ideas with a grain of salt!!
I use one for a plant stand on my back porch...just dropped a pot of ivy into the top opening.
The 20 gallon one, I use to catch rainwater at the bottom of a drip cap.
One holds rolls of gift wrap in the spare bedroom closet.
The 4th one holds a collection of canes and walking sticks beside my back door.

That's a good start! I have one too. Just a "stand" for now. I would love to read of other uses that you creative people have come up with.

I have one that I covered in fabric. Cut fabric in small pieces and glue at random on can. It was blue background with black cows. Tied a blue ribbon around the top. I have it in my den with a cow sitting on top. I also have some that I painted.

You can also do a search on "milk can" on eBay for ideas.

I use mine for umbrellas

I wish I would have kept the one I had years ago. I think I had tall dried botanicals in it. Would love to have it for umbrellas now.

I've seen one made into a stool by attaching a wooden cap that had been covered with foam and fabric.

They'd also make a great lantern. Pick a design and draw with permanent marker where you want to drill the holes. They're too big to put in the freezer so if you filled them with water in the winter then when it's froze you can drill the holes. Bring it and let it drain out in the sink or drain in the basement drain. Add a jar candle and be very careful when lighting.