1) Change most of your money into Yuan, it is the easiest and best for day to day transactions.
2) Carry the rest in US travellers checks and you can change it into Yuan if you need later on.
4) Haggling is a must. In most places you can get to at least 50% of the asking price and in the markets you can usually go even lower. The best technique is to haggle down until you are getting close and then say no, still to high and walk away. I guarentee they will stop you with a lower option.
5) Don't pay any extra taxes to a guy at the airport. There is a ring of guys in suits that will try to convince you they can check you in before the desks open. They will tell you there is a tax and give you an official looking receipt. Just walk away and they will leave you alone.
6) Lots of things you should do. Don't miss the Great Wall, if you're in good shape there is a great walk from Jinshaling to Simitai. You can book the tour at any of the trip depots around the city.
Obviously the forbidden city and square are worth visiting. Visit Mao, it will take much less time then you'd expect, but Forbidden city will take much longer than you'd expect so it balances out.
The night market is a neat (if a little touristy) place to visit for dinner.
Finally if you get a chance check out an acrobatics show, you won't be dissapointed.
Have fun and don't be too concerned about walking around the city at night or during the day, it's safer than most people think.