I don’t know how you commemorate the anniversaries, because I haven’t had one yet, but for my Confirmation at Easter my sponsor gave me a First Communion Rosary.
I think a number of things would be appropriate- here are some ideas of varying expense:
A medal of the person’s chosen patron saint
A prayer book such as “Blessed Be God” or “The Blessed Sacrament Prayerbook”
A small statue of Our Lady or Our Lord
A Miraculous Medal
A St Benedict bracelet
A Rosary bracelet
One of the small “May Day by Day” or “Augustine Day by Day” books
An Icon of their patron saint
A subscription to the Magnificat magazine
A CD from Lighthouse Catholic Media to help them grow their faith
A book by Scott Hahn or Mathew Kelly or Peter Kreeft to help them learn more
A wall Crucifix
The Catholicism DVD series by Fr. Robert Barron
A selection of prayer cards
A book on selected writings from the Church Fathers
Or even simply offering to continue acting as a faith mentor for them by meeting regularly.