You could put things like fabric/color swatches, vendor contracts, to-do lists for a particular week, notes from things you recently did (venue tours, vendor options, dress options, etc), pictures from each stage of wedding planning, words of encouragement, contact information etc.
I loved Lisa’s ideas. You could also add business cards. You can write a few notes on a notecard after talking to a potential vendor to remind you what was discussed. Then you can compare them later and choose the vendor that works best for your big day!