Mock Trial is a blast, but I've been lucky in that neither my high school nor St. Thomas take it too seriously. (Although we do quite well at it.*) If your team takes it too seriously it isn't as fun but I would recommend trying it out along with Model UN.
*For some odd reason, our delegation to Model UN always does really well and tends to form the center of most resolutions despite several members being buzzed, if not flat-out hungover.