You don't have to get fancy - just tie a brightly colored bandana (or two, or three LOL) anywhere on the scooter. You can also tie brightly colored/patterned grosgrain ribbon as well.
If you want to attach things, remember Velcro OneWrap is your friend (It's Velcro, with the hooks on one side, and the loops on the other, so you can loop it around things, and it sticks to itself) as well as "cable ties" (aka "zip ties").
Some people will use beach towels to cover their seat, and I have seen people put everything from bicycle handlebar streamers and bells, to battery operated LED lights, to stuffed animals that have been attached. General rule of thumb, as always, is don't put anything on there that you will mind if it gets "borrowed permanently" by another Guest.