The extravagant bar mitzvah drives me absolutely nuts. Call me old-fashioned, but there's always the tried-and-true classic fountain pen. But since it seems to be an obligatory social custom in some circles....
Obviously Israel Bonds have been a mainstay for a long time. Judaica is a good one: mezuzahs, kiddush cup, seder plate and such. Though seder plate is usually more of a wedding gift than a bar mitzvah.
If the kid is non-Ortho he might not have tefillin. They're kind of pricy (probably starting around $400) so maybe it's for a few families to go in on together.
Not a Jewish gift at all, but I'll tell you the best gift I got: a distant cousin bought me about $200 of shares (split between two companies). The week after, he came over to my house and gave me the 101 on how the stock market works. Taught me to check it every day, keep an eye out for news about the companies and such and make decisions for myself about when to buy and sell. I followed them for over a year, selling one around 2 months and the other around 14 months - a fantastic experience that taught me financial literacy and responsibility.
I also got some lovely candles.
For a bat mitzvah candle sticks are a good bet. They probably won't have their own set and it's a nice thing to have, especially if they're pretty. Even if they don't use them, at some time in their lives they may want to light shabbat candles and some small, portable sized ones of their own is a nice option. Still I have seen some really strange bar/bat mitzvahs, and some of the invitations that get sent can be "interesting," especially some of the more decorative one, the best though had zebra print on it (reformed family).