Walking tour in Bangkok

Dear all,

Planning to take walking tour in Bangkok, any recommendation for the good one?

We are only two of us, don't want to book a private tour.
Prefer to go with a large group with other tourists. I am searching like Sandeman's Tour in Europe which offer free walking tour and give tip to the tour leader at the end of the tour.

We will stay in Glow Pratunam Hotel. Planning to see Grand Palace, Wat Phra Kaew, Wat Pho temples, take a boat ride in Chao Phraya river..

Any responses are welcome! Many thanks!

Melina