Here is a list of what I keep on hand for those "just in case" moments of spontaneity. See how it compares with what you are sporting around in your junky trunk:
Baseball hat & Bandanna (for last-minute hikes & messy hair mornings)
Blanket (for the park & it covers the backseat when I drive 4-legged friends)
Board Shorts (hot tubs & pools are everywhere)
Camp Chairs (2)
Climbing Shoes + Chalk bag
Cycling Shoes (in case I ever make it to a Spin class again)
Disc Golf (set for 2)
Grocery bags (reusable ones for shopping & plastic ones for dog walking)
Flip flops (spare pair for poolside)
Hoodie (chilly canyon nights)
Longboard (Pintail, of course)
Magazines for wasting an afternoon working on a pre-cancerous tan
Running Shoes + socks
Scrabble Game (don't ask)
Tennis racket (2) + balls
Tool kit (only used 4 items in it so far and never anything to do with my car)
Towel (see Board Shorts above)
Volleyball (outdoor + it glows in the dark too!)
Somewhere in there I have a spare tire & a jack too.