If you only have two days in NJ consider this itinerary.
Day 1 - If it's nice go to the Purple Mountain and up to the Purple Cloud Lake. It costs 70rmb to get in and about 20rmb in a taxi to get there. You can see the Ming Tombs and go for a walk around the lake and see the Mausoleum at the same time.Spend most of the day here and then go back to the centre and go to Fuzi Miao for the early evening (Confucius Temple). Go for a boat ride if you like or a rickshaw ride. In the evening depending on your age you can go to "Bar Street"- its referred to as "1912" (on Taiping Beilu) where there are countless bars selling expensive and noisey beers. Here there are some quiter restaurants which are good fun. In the Tepanyaki restaurant you pay a set price (150rmb) and get all you can eat cooked infront of you by your own chef.
Day 2 - The Museum of the Nanjing Massacre should be seen (unless you have children - they would definitely have nightmares if you took them). Lion Hill (on bus route 34 or by taxi) is near the Yangtse River and has amzing views over the river and city. It costs 40rmb to get in and you can pay another 3rmb to take a lift through the rock to the top so saving you sweating your way up steps. It's a really wonderful place, so clam and peaceful and the grounds are beautiful.
The old walls are everywhere so you don't need to make a special effort to see them. There are numerous "gates" into and out of the city which you will pass through on countless times as you travel the city. A lot have been rebuilt with only patches of the old brickwork remaining.
Nanjing grows on you! It's a nice place to visit and in which to live.