I'm flying to Bangkok and wondering if it's really necessary to book hotel before we arrive or if it's fairly?

easy to just get one once we're there. We don't need to stay anywhere fancy. We really don't plan on being in our hotel very much. We are going at the beginning of April. Also what is there to do as far as night life? I'd like some names of places so we don't miss out on the great places by winging it. thank you