I will cut the front off, and if it's not written on, use it as a quick little notecard. I will then stick it in an envelope that might only have had my name (or "Mom") written on it, cover the writing with cute stickers, and mail it off.
If the back of the front side has writing, I'll glue it to new cardstock to make a "frame" for a new card.
When my kids were little, I'd cut up cards into puzzle pieces and let them put them back together.