Simple Examples:
* Silly Snakes Slither Slowly.
* Sun Shines Strongly.
* Sleepy Squirrels Snuggle in Sleep.
More Complex Examples:
* Sea Shells Sparkling in the Sun.
* Strange Sights Struck the Sailors Sailing on the Sea.
* Silver Swan Swims Softly.
You can use your imagination to come up with many more alliterative phrases using the letter "S"!