Lots of hot coals is the secret to cooking on those. Since they are all opened up, you need a big enough bed to last a while, you can move the rack down as the coals burn up.
I was big into SCUBA diving in the mid to late 90's. We spent a lot of time at lakes and quarries in the NW Ohio, NE Indiana & SE Michigan. We usually dove from public parks, so we delt with those grills. We would actually take our own firewood to get a big enough bed of coals. We'd build a campfire in the grill, get geared up and go on an hour dive. By the time we got back out of the water and undressed, the coals would be just about right.
It's not going to be perfect cooking, wind is a b!tch, sprinkles/rain is worse. We had a home-made grate, we carried with us, that we threw on top of the groady thing that is usually chained to the grill. If you don't have one of those, tin foil is you friend.
We cooked everything from steak to hotdogs, including fish a couple of times, on those park grills. It's not going to be gourmet everytime, sometimes you'll eat burnt meat.
Practice makes perfect, beer makes it more perfect while you get practice.......
Good luck
Tim