The rainy season doesn't really hit until august, and even then, unless you are extremely unlucky it never rains for more than a few hours.
Mid April is Songkran, which is basically one crazy water party, everyone in the streets with water cannons. Definitely a fun time to come. April is also the hottest month I believe, but if your on vacation you're probably cool with that.
http://en.wikipedia.org/wiki/Songkran
