Answer:
I keep things green and keep kids happy during summer time.
What goes up and down stairs without moving?
I'm as simple as a circle worthless as a leader; but when I follow a group their strength increases tenfold. By myself I'm practically nothing.
What can fill a room but takes up no space?
I can swim but never get wet. I can run but never get tired. I follow you everywhere but never say a word.
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.