Find a pond or spot on a lake that is easily accessibly. Not sure if he's able to walk or not but be sure to bring one of those Coleman fold up chairs or something he can comfortably sit in/ stand up from.
Around me there are plenty of lakes that have coves where the public can go to hang out/ fish. Picnic tables there. so drag one of those over and let him sit on that .
IF he wants be adventurous find a lake that rents out pedal boats, you pedal !! haha