It's great that you're doing an experiment on this. I will list suggestions below your experimental steps.

1) having 15 test subjects throw 5 tennis balls each into a hoola hoop / bucket 10 meters away.
--> Increasing the numberof throws per subject will help you get a better sense of their average accuracy. Maybe 15 throws each? You should also record the order of the throw (to account for them getting better/worse accuracy over time).

2) spinning 15 test subjects on a swing for 30 seconds and then having them immediately throw 5 tennis balls each into a hoola hoop/bucket 10 meters away.
--> See above suggestion.

3) once the subject gets off swing I will have a timer set to time how long they feel dizzy for and will stop the timer when they notify me they no longer feel dizzy.
--> Sounds good. You can correlate that with the throw accuracy.

4) recording and comparing results
--> Sounds good. How are you going to do this comparison?