Nitin, Since you did not mention the days ( i.e weekends or weekends) and the month of stay, the answer to your query is a little long. If ur stay includes a weekend, you should visit the chatuchak market.(Its open only on the weekends). Its mostly apparel, footware . Bargaining is a must. A little attitude works.Also a little knowledge of common thai salutaions helps.( lots of brochures available at suvarnabhoomi airport). More than apparel,footware and purses ( available at a decent mall in any indian metro)a better purchase is the sony lcd/led available at in Indira place. You could see a lot of indians queueing up there. Pratunam is teeming with shops. MBK mall is worth a visit. so is their ocean park and madame tussads and the official buddhist monastery where the remains of previous kings are kept.Pattaya is only 2 hours away from bangkok and is a one day tour. If you are a non vegetarian must remember to taste the street food.
I suggest Indra market. Lots of shops and very good price. If you are staying during weekend, Chatu chuk market is excellent for shopping. I found MBK mall is only for just visiting. I could not buy any thing there.