Over the years of my trips, I have kept fewer physical mementos in favor of my digital collection. I (try to) log and capture everything digitally (with photos) during my trip.
I print as few things as possible at the start of each trip, using online versions (in DropBox for access on or offline) whenever a hard copy is not required. I bring envelopes from home for the collection of receipts, ticket stubs, etc - one per country or currency or week or town, depending on the geography and length of the trip.
Ticket stubs, etc: I (try to remember to) take a photo of each city map, ticket stub or sight brochure the day I go to that place or event. That becomes a part of my digital collection of memories of my trip, timestamped with the photos so it's already in the correct digital sequence.
I log expenses in an Android app, which allows me to snap/upload a photo of the receipt. This gives me an easy record of the total cost of my trip, by category. (I keep hoping the Quicken app will improve to support foreign currencies so that I can switch to using it. That would be digital delight!)
I keep the physical mementos until I get home, where I confirm I've captured it all and decide there what to keep, or mostly discard. I choose a few things to display on my magnetic "My Travels" board.