If you're looking for something durable and you don't need a huge amount of storage,
a flash MP3 player is probably your best bet. I have a few suggestions for you, and I think you'll be pleased to know that you can find each of these for less than $150:
The Apple iPod Shuffle is really easy to use, and there are several accessories available that will allow your niece to customize it to her liking. The downside is that there is no screen and no real features to speak of.
The MobiBlu DAH-1500i comes in several colors and can be worn necklace style with the included lanyard earbuds. It's really tiny yet packed with features such as an FM radio. The interface isn't the easiest in the world, but it wouldn't be tough for a tech-savvy user.
The Sony NW-E407 is a durable MP3 player with a unique, stylish design and a fun control layout. The main body is surrounded in a thick, shiny plastic that gives the player a nice feel and a cool look. However, the software that you must use to transfer music to this player isn't so great.
If your niece likes to buy and download music online, check out our music compatibility wizard to see which music stores are compatible with these players.