Whenever you stay at a hotel, take home the disposable shower cap. If you don't travel much, enlist friends and family to collect them for you. If you need a bigger supply, look for Covermate Stretch-to-Fit Food Covers at the store or online and select the Large or Extra Large size and buy a box of them.
This solution will not keep all of the rain out -- you won't be able to splash through puddles without getting some water in your shoes. Your socks will be damp. But I found that it kept my feet from being squishy wet during a four hour half marathon with over an inch of rainfall.
You may also shop for reusable waterproof shoe or boot gaiters if you often walk in rainy weather and don't want to wear waterproof shoes.
