Is this a one day thing? If yes, for 18 year old boys, I’d suggest cargo shorts for extra masks, their wallet with excess items removed, and their phone with at least a wall outlet charger or a back up charger. And a bit lecture on sunscreen in the morning before they take off to the park.
If this is a multi day trip, then maybe a light weight string backpack for each of them for disposable rain ponchos, extra masks, travel size tube of sunscreen, travel hand sanitizer, and a phone back up charger.
In both scenarios, they need to be taught how to access first aid to clean up any cuts in the park and asked if they want to carry a reusable water bottle or not. Some guys that age are all about reusable water bottles and will carry one all day. Others are simply going to buy a cold drink each time they want one. And well, enough money to eat and drink as they want during the day.