Ayuthaya Province looks good! will definitely consider. Thanks!
I think chao phaya river is also good for taking sunset...
Be careful of the busy streets, either they will cut your bag or pickpockets.
Put your dollars and cents on your front jeans pocket.
never put important stuffs in the backpack, it will be good to use a pouch, or a sling bag and have it in front of you. even better if you can get those thin travel pouch which you can use it under your tshirt.
I always reduce the number of time I need to take our my wallet or important documents when walking around, by putting some extra cash in my pockets at the front as like you mentioned.