How much are you looking to spend?
Is she the kind of kid that will do things with other people, or does she prefer to play by herself?
Admittedly, I was an usual kid, but a Hoyle's Complete Rules of Card Games (which includes solitaire games as well as games for two or more players) and two decks of cards was good for me for hours upon hours of amusement.
I collected Breyer Horses ... they're still around, but you might want to avoid dust-ables.
I was looking at the amazon.com listings for 8-11 years olds, but honestly, nothing really stood out. Scrabble is timeless, but a poor choice without someone willing to play with her.
I have four very expensive Native American Flutes, but I keep
one of these in the trunk of the car in the event of a flute emergency. These plastic flutes have pretty good tone for the price, and they are very durable. It's musical, requires only a small amount of musical skill, but has a high coolness factor. It can reinforce a lot of the skills she's learning in piano, but without the drudgery associated with piano practice (I took piano, and a bunch of other kinds of music lessons, so I know what that's like).