Warm layers that you don't mind getting wet - wool, pile or polypro sweater/shirt and shorts/swimsuit to go under a wet suit.
Old sneakers you don't mind getting wet (or neoprene booties, if you have them). Wear wool socks inside these, not cotton! Tuck shoelace loops and ends under the crossed-over parts of the laces between the eyelets, since loose loops could get caught on your footpedals and trap you inside the boat during a capsize!
Sunscreen
A few dollars for gas (if paddling off campus)
Snacks and water
Towel and change of clothes
Nose/ear plugs (optional)
Rain coat if it is raining - to finish signing up for the club after the lake session