karen- here are my thoughts.
the wedding attire- honestly, I think this depends on what you are actually wearing. Since you are having a slideshow, I don't think you should, however may just wear white.
Favors- what if you bring back some crafts from tulum? Maybe from the local artisins (sp?) and attach a label or thank you tag. I remember last time I visited the area, there were lot's of affordable crafts when we stopped in the small towns. Maybe you should contact Sol Tamargo, she's located in that area and may give you some advise.
Centerpeices/flowers- I think you can do with out flowers. Or if you do, maybe incorperate some of the flowers that you are using in your bridal bouquet (from your actual ceremony). Also, since you both are into photograpy and crafts what about lining picture frames with pics from your wedding on the tables? Or maybe making Luminarias with pictures http://bestdestinationwedding.com/forum/t1624
Cake- I think this is a great idea!
Hope it helps!