You are having two options. Netbeans and Eclipse ,But i don't think both are lightweight ,they are heavyweight champions actually. Considering J2ME netbeans is very good platform to start with. Its having easy to set up configurations for J2ME and nowadays everything is bundled to netbeans package.
Then you have Eclipse which is equally good and widely used .Eclipse is having new MTJ(Mobile Tools for Java) package for J2ME which has made many tasks easier than before. Its emerging as a ideal IDE for many languages and frameworks. As an IDE you have more benefits for Eclipse compared to Netbeans .
Anyway you have to try both select the one which you find comfortable .
