It's an interesting question, it's hard to answer without examples. Let me tell the difference. “Jin tian ni zuo shen me” means what are you going to do today. “Jin tian ni zuo shen me le” means how did you spend your day today. “ni zuo shen me ne” means what are you doing now. But “le” is not always used in past tense, we use it to emphasize the result, effect or influence of the action. Is this more clear?