This might sound silly but somtimes I use temporary tattoos to decorate stuff and then cover it with some kind of seal, perhaps modgepodge would work. Im not quite sure though since Ive never used it on an item thats going to be washed. If it does work though, then your possiblitys would be prettymuch endless. Photos, stickers, pressed flowers, etc.. and if it doesnt im sure theres some kind of sealant out there you could use. just ask somone at michaels or joanns or whatever is near you.
Are they the stiff kind of wicker place mats or the bendable kind? I cant remember where but i remember seeing somone fold a placemat like a U and sewing fabric between it and a liner on the inside to turn it into a purse. If that makes any sense You'd just need to add a few straps to it. As for all the rest..
What shape are they? You could get some clear plastic thread and stitch them all together to make a rug/picnic blanket. Really though I think it would depend on what kind of placemats they are as to what all you can do with them. Do you have any photos of them?