When you have unlocked the crossbreeding function it can be used to make new interesting species. Although this feature is expensive it is well worth it in the long run as the new animals will keep your zoo status high.
Money and Experience
Do not waste your time with anything that does not generate money or experience. Only purchase decorations, support structures, walls rivers or sidewalks when you are prompted to add them before acquiring the ability to buy a new species of animal.
Nursery
When you own two of every animal you do not need to waste money on buying any more of them. What you can do instead is use your nursery and breed the final animals you want. Doing this will save you money and allow you to use it elsewhere in your zoo.
Trash
Always try to clean up the trash around your zoo and inside the animal cages. Doing this will earn you money and experience and help maintain your 5 star zoo status.