Race number and pins or if entering on the day; correct money and pins.
Race - Running shoes, socks, shorts, vest/t-shirt, vaseline and depending on weather; long sleeve alternative, gloves, buff.
Post race - Normally I just change my t shirt and throw a sweatshirt and some bottoms on, but if it's been throwing it down you'll need a change of shoes, socks, underwear, t-shirt, and then a sweatshirt/fleece, pants. plus a towel/shower stuff if you plan to use the facilities at the center (i never bother) and then drink and food.
My car key goes in the little key pocket on my shorts with no problems.