With a definite time limit and little experience, I'd do something like the C&O Towpath and GAP between Washington, DC and Pittsburgh, PA. It's about 240 miles each way and I'd bet you guys could easily do an out & back trip well within your 30 days. I went one-way for my first touring/bicycle camping trip last year and I plan to repeat this spring, this time both ways, with my 13 y/o son.

http://www.nps.gov/choh/index.htm
http://bikewashington.org/canal/
http://www.atatrail.org
http://bikewashington.org/trails/gap/