The city wall was amazing. It is well worth the 40 RMB (~6.50 USD) entrance fee. Also, once you get on the wall it is another 40 RMB to rent a bike. I recommend biking around the whole wall, which is around 13.5 km (8.5 miles) -- should take around 90 minutes, give or take depending on how fast you bike.
We had Wendy as a guide from http://www.EmmasTours.com -- I recommend booking your guide before you travel with a reputable company (Emma's Tours is good, but there are other good ones too. You can check out some of the other highly recommended ones on TripAdvisor). We have kids who are in a Mandarin immersion school, so Wendy was wonderful with the kids and interacted in both Chinese and English.