The tour we attended was a lot of fun. Helmet and goggles are provided, as is a small space in each ATV for purses/bags/water bottles. I recommend arriving at the check-in time having already gone to the bathroom, sunscreen on, and close-toed shoes on. Bathroom was in a neighboring building and was only 1 person at a time when we went, so takes about 5-10 minutes for 2 people to leave the area and go to the bathroom. Sneakers/tennis shoes are probably fine, but I recommend hiking shoes as EVERYTHING will get muddy and hiking shoes have better grip on muddy soil. I also recommend bringing water is there is no water stop/area.
Mud gets everywhere (hair, face, shoulders, arms, torso, legs, socks, shoes, EVERYTHING), so wear clothes/jewelry that you don't mind getting dirty b/c it may not wash off, even clothes under the outer layer got muddy.
We had a great time! My husband took the 1st and 3rd sections. I took the middle, unknowingly, this section was the muddiest since it was at the highest elevation. We had fun slipping and sliding and enjoying the less touristy areas of Hawaii. Definitely a bumpy ride, but we were all happy screams and giggles. For those who don't like loud engines, you may want to bring ear plugs since there are a lot of windows and it gets a little loud. A thin extra layer might be nice for those who get cold easily, too.
At the end, they provide a quick rinse in an outdoor faucet for you to wash off mud from your skin. So it's probably helpful to bring a dry and dark towel to dry yourself or lay in the car so that the car doesn't get too dirty. Could also bring a change of clothes for those who don't like sitting in dirty clothes until the next stop.
Thanks again!