Which is a simple sentence?

 - Javier stacked a crate of oranges on top of the crate of lemons and checked both crates off his order sheet.
 - Although she left her house early, Clare barely made it to the train station in time.
The first sentence is the simple sentence. It is a single independent clause.
Javier stacked a crate of oranges on top of the crate of lemons and checked both crates off his order sheet.