This isn't a relaxing trip! Fitting in this much in 3 weeks involves a lot of unpacking, repacking, travelling and not a lot of sleeping! Take a small suitcase - something you can carry easily by yourself, our tour leader ended up carrying several of the group's luggage as they weren't able to carry it themselves (sometimes over long distances and up and down stairs at stations). Also make sure you have an easy to carry day pack for overnight train journeys and the night in Tongli as you can't take your main luggage into the town.
On our first night, Dragon gave us some information about China and what to expect during the trip, one of the most useful phrases we learned was "This is China!" Every time we came across something totally alien or which seemed wrong or crazy, the group would remind us that "this is china" and just accept it! The food was a lot better than I expected, but some of the group had specific dietary issues which were quite difficult to manage and they often didn't have much choice to eat.
Make sure you budget for all the optional extras - you're unlikely to be here again so make the most of it and do all the extras you can. The £400 suggested in the trip notes is a little light - I spent nearly double this. Also don't take travellers cheques, they were difficult to cash. Also try to make sure you explore on your own as much as you can when there are no activities arranged - head into the back streets and away from the tourist areas to discover the real China - it hasn't changed as much as the brochures would have you believe! Try the real local food where you can from roadside stalls or cafes - it might look a bit odd and you might have to point at what others are eating as there are no menus, but it's a great adventure and you might even get something edible!
Above all, go with an open mind, and enjoy everything China has to offer. Have a brilliant trip