Pratunam/ Platinum Mall is great place to get the stuff. Of course, Plat Mall doesnt allow for Photographing as announcement is made.
Suan Lom market is nice to go but it is expensive then Chaktukchat Market which you get much cheaper stuff there.
For Tuk-Tuk, take it to have the thrill. Rem to bargain for price and make sure they dun drive you other place. The driver drove u to a shop, ask you to go in the tailor shop and walk so that he can earn free petrol. Just beware of that.
Taxi - Usually Green Yellow cab is safer. Most of the time they allow meter.
Other then that, the rest had mentioned the great places