Giving is the perogative of the giver. Consider their alternative. A charter trip probably would have cost more than they gave you and they probably had alot more fun. They probably would have felt like they were imposing if you didn't accept the money. If you think it is too much they obviously didn't. Since you didn't ask for it make them stay happy and keep it. Maybe acknowledge receving the money and tell them it seemed like too much but you will accept it knowing they had a great time. Move on.