If you are looking for cool things to do in Bangkok, then consider getting away from the crowds that are just seeing the sights that all other visitors come to see. Maybe take in some of the local history, or perhaps go shopping at a market where only the people of Bangkok usually do their shopping? There is a lot to choose from.
Just like you know your own home-town well, your host will know Bangkok well and will know just where to take you. Having somebody with you that knows the area so well will help to make for a great day out.