E.B.
A collage of photos of them together from over the years. My daughters have done this for friends that have moved. (and just for fun birthday presents).
Stationery is fun-I still have letters from a friend when I moved to a different town 50 years ago.
A phone card is a little dated in this world of cell phones with free long distance.
Maybe just take her out for one last night on the town? Or a mom/daughter lunch? Since they are moving-mom might be frantic-maybe bring lunch over for them or the mom might appreciate a mid day break.
If they have cell phones or Facebook then they will keep in touch better than you ever imagined.