In addition to using photos, you can commemorate a trip through receipts, brochures, business cards, menus, and even food wrappers. Pressed plants, feathers, or other natural tokens are also evocative.
If you buy a new road atlas, save the old one for this project. Also keep the free maps given out at rental-car offices, hotels, and tourist destinations.
Resources: Kaji top-load album and Series H refill pages, kolo.com.