The Chao Praya river snakes through Bangkok and along the way it creates a few islands. These islands flood frequently and so they're designed green areas. I've visited Bangnarmpheung island before to see the weekend floating market, and the fireflies, this time I'm visiting Ko Kret further north.
Getting to the island is simple enough, ferries run every few minutes from the east bank of the river. There are two temples here, the ferry runs from piers at the two temples, and if you get there early enough you can park at the temples.