I would suggest sitting on your flip flops. I thought I could hold mine with my toes. I'd ridden several times and thought I could do it. The TM didn't say anything to me about taking them off. The ride stopped at one point and I reached over and grabbed them off my feet. I really think I would have lost one if I hadn't had a chance to grab them!!
The other thing is that things seem really, really close. When I'm not wearing shoes all I can think of is gripping my legs to the ride car so nothing touches me!! Just fair warning. Have fun!!