The ideal Bar / Bat Mitzvah gift should have the following qualities:
1) It should be something the child will use for the rest of their life or at least for a long time.
2) It should have some symbolism of Judaism and not a gift without any connection to the religious experience of the Bar / Bat Mitzvah.
3) It should be something the child will not get fifteen of from other people and will simply try to exchange or return.
That being said here are some of my top choices.
Siddur / Prayerbook with their name engraved on it:
They will use it over the other siddurim they have or will receive. Another option would be to buy them a machzor (prayerbook for holidays) instead of an everyday siddur or perhaps for girl a book of Pslams (Tehilim)
Jewish Reference Books:
Based upon their level of observance and skill, you should try to get the reference books that they will use forever whether it is a Jewish Encyclopedia, set of the Mishnah or perhaps Chumash with commentary. Again, inscribing with their name will assure that they will use over others
Personalized Jewish Artwork:
There are many creative artists (maybe you are one yourself) that have produced special artwork for gifts. Some are similar to time capsules using pictures and mementos of the event. Others are framed pictures either of the child or something else Jewish themed and appropriate for the young adult.