Zoo Tycoon

Build a park in challenge mode. Simply just keep completing challenges. Challenges are for the most part very easy, they can be as simple as delivering food to your concession stands or taking a picture of a certain animal. Challenges also include things such as releasing an animal or a certain number of animals into the wild, or breeding a certain animal. I have found that the more you build and the more you have going on in your park, more and more challenges will arise.