Easiest to pack, and quickest to use: bright, cheerful lengths of grosgrain ribbon, or colorful bandanas can be tied on to handlebars, armrests, etc.
I have seen people use all kinds of garland - everything from Christmas tree garland to Mardi Gras beads - to decorate the baskets on rentals.
There was one time when I saw a "generic" rental seat cover - think of something about the size of a beach towel - that was draped over the back of the seat, and down the seat itself to create a colorful temporary seat cover. Very cute!
Some folks hang colorful backpacks off of the seat back. Vera Bradley packs are popular (in Disney prints, of course LOL) but often you will find that WDW has a "special" backpack on "sale" for $20 with a purchase in the shops.
I have also seen folks make a small laminated "license plate" (Disney themed, of course!) that has their last name, or their nickname, or some other word that is meaningful and hang that off the back of the rental seat.
You can also use small battery operated lights at night - just please DON'T use flashing lights that could cause a seizure, or lights that would be aimed into the eyes of little Guests who ride in strollers! Everyone deserves to have a great time at WDW!