I like to use decor that is useful at baby showers. The first thing that came to mind was rattles, teething rings, etc. attached with ribbon to the chairs. I also thought of baby blankets, diaper burping pads, bibs, etc. draped over the chairs for color as well. Again, everything is useful and can be given to the expecting couple. There are many wonderful ideas for making burp pads, diaper changing pads, etc. on Pinterest that you may wish to check out.
Here are a few ideas: http://www.hometalk.com/search?filter=baby+shower. Also search on Pinterest. Here's some pretty ideas from Martha StewartL http://www.marthastewart.com/340485/baby-shower-decorations/@center/276963/baby-shower-ideas