I did illustrations on the outside that referenced things about the couple (their hobbies, hometown, colleges, etc) and on the inside, we came up with prompts for the guests such as "advice for the bride and groom" or "places the bride and groom should travel to in their lifetime."
The best thing about the project is that materials-wise, it's relatively inexpensive. We bought two pads of 9x12 bristol online for about $4 bucks each and I used 3 pens for the drawings: a thin pentel unisco micro-tip, a thin-felt tip for the first layer of the numbers, and then a sharpie to darken the numbers. The total was less than $20.