Let's see. If she likes that one - well, she has good taste! I recommend "Tunnel in the Sky", "The Rolling Stones" "Podkayne of Mars", "Double Star" (a good lead-up for Moon is a Harsh Mistress) and if you're being really subversive, "The Door into Summer".
Sigh. I was lucky to have a dad with good taste when I was 12 and star-struck. Sounds like your niece is lucky to have you around.
The ones I got her are: Starship Troopers, Rolling Stones, Have Spacesuit Will Travel, Podkayne of Mars, The Star Beast and The Moon is a Harsh Mistress (still debating on that one - might not give that one to her).
BUT, based on your recommendation - and because I trust you wouldn't choose something inappropriate - I just bid on Ender's Game on ebay. I wasn't going to spend anymore... but... :-) books is worth it! :-) She's getting much more from me than any other niece or nephew with just those. Somehow, I feel too much empathy for her to *not* let her be a favorite this year. :-)