If the flight is on time and I calculate 1h for immigration/customs/etc (I hope it is less) and since our luggage will go through to the final destination and there is no need to check them again, I believe 2 hours prior to departure are ok to be back. So we have indeed 6:30 hours free.
Any advice on what to do these hours? It will be afternoon. Go riverside? Go to a mall? I would rather see something Thai than a mall with international shopping brands.. Maybe a dinner somewhere nice around 7-8pm?

Thanks!