I also suggest either using any of the local FT experts or using one of the companies providing taxi/limo service. You could also use CTRIP or similar to book those services (a bit more pricy, but works well)

What you want to do - a day trip to the Great Wall - is very common, and there are plenty of services to do exactly that (and quite affordable if you think that it will include a driver for a full day, a car, and all other costs.. )