I would recommend the following:
A 3-5k tournament that has 1 round at 4 different courses (probably 9holes each would be good), as a multiround tournament with various conditions concerning tees, wind, green speed. Plus a random drawing of sth, should attract some players. What do you think about this?
Apart from this, best courses for a 5k Ready-Go are:
Cabo (look at how that 1k ready go always filled up!!!)
still Bethpage, maybe you need to announce some easy conditions openly and give a random drawing to lower tiered players if they join?
Cheers, Hans (what about a 25people ready go?... :-))