There's really only 1 way to JMU from the DC area: 66W to 81S.
Honestly, there's not a lot of must-see stops along that journey. If you're a Civil War buff its worth stopping at New Market, which is about 15 miles north of campus. The battlefield, memorial, cannons, etc. can all be seen from the highway. But I'd advise just getting to Harrisonburg early. If you have time I suggest taking a stroll around the Bluestone area of campus (where the Quad is), or going into downtown Harrisonburg for a bite to eat at Billy Jack's or Jack Brown's (universally regarded as the best food in town).