Hi Ziners, Leslie, you could take a boat up the Chao Phraya
river and take the bus back. Lunch is served on board and
you view the life on the river as you cruise by.
There are bound to be excursions offered by local tour
When we were in Bangkok, we elected to take the boat ride on the Klongs (canals) which was very inexpensive, took a couple of hours, and was also very interesting. Remember that when you arrive in Bangkok it's going to be hot and humid. You might want to give yourself a day to become acclimated before doing anything strenuous - like visiting the Grand Palace.