There aren't any mega clubs downtown, more bars and lounges.
This spot actually called 'downtown' used to have a tight house DJ on the weekends. 'downtown' is kinda hidden, but on the corner of Las Vegas blvd and Fremont.
But Fremont St. and Downtown Las Vegas isn't as uptight as the strip. Might be hard to explain unless you see it. And SoWhat is right, on spring nights you can just hang out ON Fremont St.