All these are amazing suggestions, thank you!!
I’ve done Kiva loans before; I may share that with him and see what he says. We’re both intrigued by the Canadian train trip (which, according to my most cursory searching, can be combined with an Alaskan cruise…) and the Panama Canal trip. I think I may have gotten an “ooooh!” with the Tuscany trip. 
We definitely want to make sure whatever we spend the money on is something that honors his mom and will remind us of her for many years. I have to admit I personally like the Kickstarter film idea.
Keep the suggestions coming, please!