I saw ศาลเจ้ามูลนิธิธรรมกตัญญู (The Santi Chai Park (สวนสาธารณะสันติชัยปราการ) and Pom Phra Sumen Fort I viewed when I took the Chao Phraya river taxi so I got off the river taxi at Phra Athit pier. On getting off here you walk down a tight alleyway which is full of 'those tourist bits' which can be quite entertaining. Walk down this alleyway onto the main road and turn left and you will be there in 5 minutes.
This is a very small park and is close to the Khao San Road and you are able to see the Rama VIII bridge from here. I believe that there also used to be a sugar factory on this spot but I could find no physical evidence.
Then in this park is the 'Phra Sumen Fort' - and there not many of them left in Bangkok -
from the original 14 that were built in the reign of King Rama I.
After this view then visit Pipitbanglamphu - the Local Heritage Museum - this is their Facebook Page. Unfortunately I did not have time to go here - maybe next time?
As an aside I believe that the idea of the fort in the park translates as "the park with a fort that symbolized the victory of peacefulness". Then there are apparently two important trees but I did not find them - Lam Phu Tree and Papal Tree.