Write down a list of things that you want to do with your daughter (cinema, zoo, swimming pool, beach, Cheesington World of adventures etc. Obviously it depends on age of daughter so your list would be relevant to her, mine is an example) and also a list of things that you'd like to buy her (cd, dvd, dress, doll computer game etc.). Then book the tickets and buy the pressie and tell your daughter that you've done so and when you plan doing the activity together.
Your daughter will love you for it.
It comes with the added advantage that it is the best way of sticking the knife in the ex
You win the psychological warfare round - plus you get to see her face as you give your daughter back after having an amazing day with her dad.
That is priceless.