Answer:
   
   The following are some of the ideas that have been suggested:
     * Jewish books involving torah study.
     * Ceremonial objects that allow observance at home: kiddish cups,
       seder plates, havdalah sets.
     * Ritual objects, such at tallitot.
     * Gift certificants for Jewish bookstores.
     * For girls, some communities give items for the dowry: sheets,
       towels, etc.
     * Fountain pens. :-)
       
   Remember that the gift is a celebration of coming of age. The present
   should be useful throughout the individual's Jewish life. Argues well
   for a fountain pen, doesn't it :-)
   
   It is a good idea to have the child register at a good Jewish book
   store so as not to be flooded with duplicate books or gifts. Gift
   certificates are nice in that they allow multivolume sets to be
   purchased without having to know the volumes that the child already
   owns.
   
   Lastly, you should not bring your gift with you to the ceremony (if it
   is held Friday evening or before sundown on Saturday), especially if
   you know the child is from a traditional Jewish household. For those
   that follow traditional halacha, carrying is forbidden on Shabbat
   [there are some exceptions, but as those vary from community to
   community, its just best not to carry at all].