First off, it's awesome that you kept in touch with your mentee even after you moved away - I can't imagine how much that meant to that person to know that you still cared and were still there to support your mentee. I think the postcard/letter idea is a great one, as is creating a photo book of your activities. That way, they have a tangible symbol of your relationship AND an easy way to make sure you stay in touch!
Your post really resonated with me. I think it's the not known that is so difficult for mentees when ending a match relationship - when they don't have a reason to explain why their mentor is suddenly not there. Is the mentor OK? Are they just busy? Or, do they not care anymore? Your point about being truthful can't be overstated; having some tangible reason to understand what happened between the match can really help mentees sort out their feelings about the relationship coming to a close.
