Give yourself the time to fall in love with Bangkok
Most of the people I met only spend 2 days in Bangkok and spend most of their time on Khao San road. I spend over a week there and I love this city with all my heart. You need to walk away from the touristy spots.take public busses, public boats, eat streetfood where you can and yes party on Khao San Road is a must. Drink buckets until you think its a good idea to eat dried scorpions or get your nose pierced, talk to strangers and have the time of your life.
Insider Tip:Be careful with Tuk Tuks. I stay away from them in general. It's fun to take one once but they will scam you 100%, no matter where you go they will try to get as much money out of you as possible and sometimes even take you to some shops where they force you to buy stuff. It's much cheaper to take a taxi (insist on the taximeter) or the local subway / skytrain.
The weekend market is awesome. It's massive and you can get EVERYTHING there. It's also a lot cheaper then other markets or Khao San.