Any particular sports in mind?
From the top of my head you could take on the team name from your state or local colleges ( UCLA are Bruins) like " Bruins Know How To Bring It!"
I like pennants across the top of the room with months of the year and students birthdays on them
Reading Dugout comes to mind
Reading Is a Slam Dunk
Some great ideas are listed here but the links may be outdated.
Stumbled across this and wanted to share . (great for 5th)