Sennheiser PX 100
Glen Reynolds repeats a past recommendation for the Sennheiser PX-100 headphones, and I want to give that recommendation a big ditto. I have three pair at home and love them. They are inexpensive, rugged, sound great, and fold up cleverly (though there is a learning curve to getting them back in the case).
Hooray! I have these. I don't bother folding them, though.
I bought these for both of my teenaged daughters (who use iPods often). They quickly got the knack of folding them and putting them into their cases (and then shoving them into backpacks). The sound is very good for portable headphones (though they can't match the sound quality and noise blocking of my Etymotics ER-6 earphones).
My wife bought these, and the recommendation is supported.
Important factor: Can they handle sweat if used when exercising? Your little sister is quite the sweaty one and needs something that can handle that AND be comfy for long runs.
I've had a pair of the white ones for years and still use them. I did replace the ear pads after about three years. I fold them up all the time since I travel with them.