This is actually an old Dutch tradition. Basically the idea is that you have a wish tree/glass bowl/jar and some note cards or tags next to it. So your guests will take the note cards and write a wish or a piece of advice and hang it on the branches,or put them in the bowl. At the end of the night/day the newly weds take the cards and make a scrap book or an album out of them. I am sure this will be fun to read in the morning after breakfast in bed when you are honeymooning.
I actually remember singing under a big tree with my friends as child at Arusha School 'wish wish come to me' we caught the leaves falling and made a wish then we let it go. Maybe wishes do come from trees after all,Oh well!
You can have different place cards like below,you can even have them in color codes