I know it's not the same as a last session, but I'm giving L two gifts: one to mark our 2 year anniversary and another for coming back together after her vacation. The anniversary gift is a lantern with 3 of our quotes and a picture on each side. The project I'm working on is a scrapbook about our relationship: things about us, pictures of her office, things we both find beautiful, things I've learned from her, etc. Maybe you could give her a gift like that to "mark" the final session?