Expand the farms to support more people. Search for more crops to plant and diversify our diets. Construct rafts to explore the river.
+1 this, but use the rafts to fish in the ocean, and construct more permanent housing than tents. Gather stones and build a Shrine, because religion is good. Also, search the area for any mountainous areas, so we can start a mine.
Over the next half a century, you build a small fleet of rafts and use them to explore the rivers and coast, as well as to aid in fishing. Upstream of the river nearest to your village is a small lake, and a dense forest grows along the southern coast, with another river running through it. While the forest is too dense to explore much of without a dedicated group of scouts, it's a far better source of timber than the isolated groves on the plain. You also discover persimmon trees on the edges of this forest, and cultivate a grove of them by your village, which is now home to close to six hundred people. You expand your onion farms as well, and although there's hardly a pebble to be found in these lowlands, you put together a wooden and packed earth shrine with a bonfire in the center for making sacrifices to... well, nobody's agreed on a name for he/she/it/them yet, but after such a bountiful time as this, you're pretty sure it likes the onions you've been giving it.
Unnamed Village:Population: 600
Climate: Warm temperate grasslands, with a warm dense pine forest to the south
Resources: Onions, Persimmons, Fish, Prime Timber, Wood
Deities: Unnamed Harvest Spirit
Neighbors: None! Yet...