This past weekend I could've used a spare electric water pump. That will be added to my list. Fortunately for me another Mopar racer had a spare and allowed me to buy it from him so I could continue on racing.
Another good item if you have access to electric is a fan that you can use to help cool the car down or yourself.
A pump sprayer is good to have when it's very hot to help cool the car a little faster.