I would use all non-mayor characters to hoard furnitures that I find nice-looking and treat it like another home! Some people will have different sets to collect and store in side characters and some may even base it off of themes that they enjoy. Just be creative and treat it like a second home!
I think what they all have suggested is a great idea! For example for me, if i'm using the princess set in my home, i would love my other character to have the mermaid set in use ^-^ since i don't need two bedrooms in my home for my mayor haha.
You can create a personality and make a house based on that.
I have 2 other characters, a rock chick. Going to give her a hippy style house, or maybe the black rococo furniture? Not sure yet tho. And in the basement a music studio with rock instruments.
My other character is cute and pinkish. She will have a very pink girlish house
You also can make a crazy professor character or someting and make a robo room and a big library or something
You can make a hospital, a gym, a library, a second home, a laundromat, an arcade...
People have done some really creative things to make their alts' homes look awesome and act as something unique within their town.
Personally I'm going with a library and a restaurant with a downstairs arcade. Figured that covers all of my basics. I'd do maybe a little clinic or something, but I have no plans on making a 4th alt due to spacing.