Hire cars for the day. Makes life a lot easier and you won't be getting ripped off every time you try to take autos/taxis. A car for the day (with driver obv.) should cost about 10 or 12 quid (this was last year. May have gone up by a couple

).
Naturally if you're only planning to go to one or two places the entire day it's not particularly worth it, but if you plan to travel a fair bit, invaluable.
If you do take public transport, avoid buses. Just pointlessly crowded and not very comfortable either. Take autos or taxis. They *will* try to rip you off and haggle it down. They mostly all have metres and if they use them, then you are largely ok. Chances are, they will make an excuse for the metres not working so fix the price of the trip *before* getting in. The walking away trick usually does not work in my experience but then there are plenty of other options right near by so you could just as easily find someone else.
Also, with taxis, they may sometimes charge you by the metre + an additional 15% (or something) for the use of the air conditioner. This is standard practice.
Shopping wise, take a look at the back of any product you're planning to buy. Unlike UK, which has a listed RRP, India has a MRP (Maximum Retail Price). They shouldn't be charging you more than that.
Stick to bottled water as someone said above. Far safer and a 1l bottle costs about Rs. 12 (13p). Also, please please please avoid street stall food. Most people I know who get 'Delhi Belly' is because they think they'l be fine and try all of it. That is *not* a good idea. Sure sometimes it won't be a problem (generally if the stall in question has a maaaasssiveee queue and all the food is prepared hot) but if you prefer to be safe, avoid it. Go to small/roadside restaurants instead (obv. the risk still exists but I do think it is smaller).
That's my advice off the top of my head. Feel free to ask if you have any other questions.
Can't advise much on things to visit apart from the mentioned things above as I never did much touristy stuff while there :s
Oh, and if you like chinese food, go to Tea House of the August Moon in the Taj. Amazing restaurant

(well, at least I love it

). It also has Masala Art, an Indian restaurant in the same hotel. Also very good.