You can try any of the tour operators on Khao San Road (this is like a backpakers street). I am not from Bangkok but have been there enough times to recommend some things to see.

Ayuthayaa...the ruins of the old city is a must. Floating Market where people sell stuff off boats is an interesting site, Jatujak market where you can get almost anything under the sun is open only over the weekends only so you got to plan this one carefully.

Massage there is a nice, clean and good place at Bangrak.... very close to Shangri-La Bangkok.

Take metred taxis and be careful about tuk tuk drivers because if tourist, they tend to bring you to some silk factory because they get a cut for their petrol when they introduce customers.

Hope this helps for now.