A bat probably isn't the greatest idea though, too big to put under the seat and would be hard to explain, although taking a ball would help the story.
Cold Steel makes the "Tire Boss" for "checking your tire pressure", although I don't know how likely that story would be believable.
Another great option is a Maglite. They're definitely not the greatest flashlights (shameless plug for r/flashlight) but a 4D cell Maglite would certainly make a good club, and nobody would question why you have it.
If you have tempered glass, a bat might not do anything at all, in which case you would need an actual glass breaker.