For shopping, I normally will go to the new Central World, then to Siam Paragon, Siam Centre, Siam Discovery and MBK. Actually just shopping in these few places, it will approx. takes you one full day. Remember to go the the NaRaYa shop at Central World, lots of people loves to shop there, their bags are cheap and beautiful. Really good for souvenirs and self-use!!! Also, another place for shopping is Emporium Shopping Centre, remember to go to their food court for lunch, cheap and delicious!!!
Night time, you girls must go to Suan Lum Night Bazaar, remember to bargain for everything you wants to buy!! At least 50% off!! I myself love to go to Suan Lum Night Bazaar more than the Sunday Market Chatuchak, because Chatuchak only opens on weekend and is really really hot shopping over there in day time.
If you would like to go for Spa, I will recommend you to go to "Let's Relax", everytime, I go to Bangkok, it is a "must" place for me. I love their Hot Stone Massage. If you have time, try it!! You can go to their website and reserve the spa first!!
http://www.bloomingspa.com/index.php
Also, another powerful useful website for you to search more information is
http://www.thailandfans.com
Hope this information can help us!! Happpy Trip!! :-)