D.N.
How about a picnic with his favorite finger foods? You could do it at home or at the park. Get some water guns, frisbees etc. Still have a nice time. When my husband injurred his back (2 different durgeries), he would not do too many things at a water park and amusement rides hurt. But he was still able to play with the kids in the yard, running lightly at times. I think we did go to a waterpark after his last surgery and he went on one of the easier slides a few times. He mostly relaxed and dipped in the water.