Cash always works. However, if you're a sponsor, a more personal gift is expected (as a previous poster said). I remember way back when I made my confirmation, all the kids got nice rings as gifts. I got a ruby ring; a boy in my class got a blue stone ring, another got a small diamond ring, someone else got a Claddagh ring (excuse my spelling) etc. I don't know if all the parents/sponsors got together and planned this, but it was funny how EVERYONE got a ring.
At my communion, all I remember getting was cash. I remember opening all the cards and the bills falling out. LOL! (I think I also got a small bible and a white rosary)