Use containers! Traditional baskets work well for open shelving, they contain your clutter and look great in a Cape Cod style bathroom. Wire baskets have that old time charm that many Cape Codders love and come in a variety of sizes. See these for sale on Etsy: http://www.etsy.com/search_results.php?search_query=wire+baskets&search_type=vintage
Bath towels can be rolled spa style or learn how to fold towels in various ways so they take up less space. http://www.homemadesimple.com/en-us/cleaning/pages/towel-folding-techniques.aspx
Many Cape Cod bathroom remodels are now using the open shelf concept for some storage. It really adds to a spacious feel and allows you to add tons of Cape Cod charm to your bath.
Check out these links for some open shelving ideas:
http://www.houzz.com/ideabooks/491219/list/Bathroom-Trends--Open-Vanity-Storage
http://www.freshhomeideas.com/Room-Ideas/Bathrooms/Bathroom-Storage/open-bathroom-storage