Comfortable sportswear and closed shoes work well for urban, land and aerial activities. Flip flops are in general not suitable for sports activities and if you have long hair the best is to tie it up.
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 summery of the confirmation email.