Sri Nakhon Khuean Khan Park and botanical garden located at the bank of Chao Praya river, opposite Bangkok River Port. How to get there: take taxi form Phrakanong BTS station to ferry near port. Cross the river by boat.
Rent bicycle (very inexpensive) and ride to park (here is 1 road only).
Park is very nice and have no entrance fee. Many rare trees have descriptive labels, you will find pool here and many nicely decorated bridges.
Recommended time to visit: from morning till noon because very hot.

Link for Google maps