Comfortable sportswear and closed-toe shoes work well for urban, land and aerial activities. Flip-flops are not suitable for sports activities. If you have long hair, it is best to tie it up during the activity.
For activities in snow or water please take into consideration that you might get additional equipment. Hence, being dressed in layers will help you to change quickly and figure out on spot in what layer you feel the most comfortable.
Please also note that some activities require you to wear specific clothes or bring along certain equipment. You will find this info in the activity description online as well as in the PDF summary of the confirmation email.