Which is a simple sentence?

 - Unless the weather forecast is wrong, you'll probably need an umbrella tomorrow.
 - Darell stacked a crate of oranges on top of the crate of lemons and checked both crates off his order sheet.
The second sentence is the simple sentence. It is a single independent clause.
Darell stacked a crate of oranges on top of the crate of lemons and checked both crates off his order sheet.