I used supplemental books that I had read to him as bedtime stories before:
Why is It Hot? (Just Ask Book)
Why does it rain? (A Just ask book)
We went outside and picked up brown and dried leaves to demonstrate summer (we haven't cleaned the front of our house yet!) and showed him the pool of rain water to demonstrate rainy season.
The top, bottom, side demonstration was a bit easy for him since he knows those words already.
Mixing time! I gave him paint -- red and yellow and showed him what happens when you combine both. Orange!
He colored on the leaves that we pasted on a drawing paper to draw falling orange/gold leaves.
Spills! I had left him to continue on his things to do and he was running out of the room holding a styro cup. He was saying "water" and I followed him back to the room and saw all his drawing pad soaked with water! Apparently he was putting water on the paint palette and spilled it all over the place. He got a good scolding from his dad after that. :(