my parents just filled a pool at their new house. a lot of work and a lot of money. plan on a couple of days and multiple thousands of dollars. the one thing you don't want to do is put the old wood and concrete into the hole. it will prevent proper drainage and it will also cause uneven and unpredictable settling. ideal fill would be a foot of clay, five to six feet of class five gravel and a foot of topsoil. you will have to fill it again in the upcoming years as it settles but it is better than taking care of a run down pool.