Well, there are a lot of comebacks you could use, ranging from humorous to more assertive...and I agree that whatever response you use would have to consider the age of the one staring, and probably even in what manner they are staring. (Those who actually stop dead in their tracks and turn completely around are the ones who get my goat the most! )

Some suggestions: (some taken from Apparelyzed's site)

** You know...walking is soooo overrated.

** Oh my God! (squealing with delight) Are you from the 'Pimp my Ride' people?!

** I'm only in it for the good parking.

** Don't feel bad for me...my other ride is a Harley.

** Sorry, it's not for sale.

** I'm sorry! Did you want to say something to me? Can I help you with something?

I have, on several occasions, and again...depending on the initial age/rudeness of that person, stopped dead in my own tracks, turned around and stared right back....and I never break my gaze first!!

With children, I see their curiosity as an opportunity to make a positive connection for them about people with disabilities. I'm much, much more 'kinder and gentler' then.