How nice to have a good MIL story!
I love the idea of a plane ticket if you can swing it - to share her with the family members she didn't see or spend a lot of time with when she was housing you.
You could also do a small scrapbook - we did one for my mother. Each person wrote a letter of what she means to them, special memories they have, fun stories or silly moments, etc. We bought the scrapbook with open clear plastic sleeves, chose background papers for each person to decorate a couple, and then slid them into the sleeves to make a complete book. We included favorite photos from over the years, made some little captions on the computer, and bought embellishments and stickers (on the various themes that emerged) at the craft store scrapbooking section. The staff at the store can be very helpful, as can crafting books from the library.
If you're not that crafty, you can make a hardbound book through Shutterfly or similar companies. It's pretty foolproof - you just upload photos into the templates they offer, add your words of love and thanks, and click "done." They print and bind it, and send it to you or to her. We did one for a relative's 90th birthday (90 things we love about her or remember about her), and each person did some individually while others were sort of group memories.
These are things she cannot buy and which are gifts of your heart.