First of all, I don't think you need shower favors. I'm the person who intentionally "forgets" them at the shower unless it's something I'm going to use, but I know there are those folks that are sticklers about giving/getting goodie bags/favors. I just got my 12th pair of holiday socks from a party. I never, ever wear them, they go to goodwill, but this one friend gives them out at every get together. :-p
With that said, two ideas: how about taking a bulk package of bath salts and putting it in cute little jars or bags with pretty ribbons and cute computer printed labels. Most likely it'll get used. I also thought about the personalized m&ms. Once again, buy in bulk and put in sweet little bag with pretty ribbon.
S.