Take ponchos, maybe an umbrella (always good to do at WDW even if you haven't seen a rain forecast). Have a change of shoes available so one pair can dry out, if you do get soaked. I love the Tune-In lounge at DHS in a rainstorm (well, I love it in dry weather, too). People don't melt in the rain, but lots of them will leave the parks, making it easier for you to enjoy the open rides. Do take shelter if there's lightning. The shops get crowded, so it's not a good time for that. Take some pictures of the rain and interesting things. It can rain furiously, so it's good to have an alternative plan (movie, board games?), but don't let it spoil your trip -- there are ways to cope. (Avoid Disney Quest; it's very extra crowded on rainy days).