LOL, I was sort of kidding on the whole 88 acres, but I tried.
Anything you put in as obstacles might cause injury if dogs crash into it. I have seen dog parks with agility equipment in and competitive agility people exclaiming in horror about untrained owners using it improperly. If there are trees dogs can have great fun chasing each other around them and you would not run the risk of someone complaining about introduced items.
Probably one thing that would be very well liked would be a pond if there is a source of fresh water. A source of fresh water probably means you need municipal, maybe state or provincial, environmental authorization to dig the pond as it would/might alter a water course somewhere. Failing that a big wading pool? I do take my dog to a large piece of crown land and the pond is very popular in summer. Also a hazard in winter and an owner had to be rescued when she went through last winter, trying to rescue her dog. Both were fine.
Sorry, I come from insurance and my OH was a provincial guy ok'ing pond construction. That's why I see obstacles that might be remote.