I'm working my way through the various ribbons and crop masteries... that's what I consider the "objective" of the game, for me. I already have the blue ribbons in Animal Shelter, Green Thumb, Tree Hugger, Noah's Ark, and Not Spoiled Gifted, and have both watermelon and artichoke masteries.
As far as I can tell, once you have the Zoologist, Noah's Ark and Animal Shelter blue ribbons, there isn't much else to do with animals (except earn coins). I already have the Noah's Ark and Animal Shelter blue ribbons, so I figure once I get the Zoologist blue ribbon (I have 265 left to get the red ribbon, then I can go for the blue ribbon) I will probably clear off the animals, to allow more room for crops, decorations and buildings.
I'm currently working on cotton mastery. I have one star and need 114 to get the second star. Once I get cotton mastery, I think I'm going to turn my attention to flower mastery and the Flower Power ribbons, though. Maybe I should just change to that objective now? I say that because it seems to me that, at this point, flowers are the way to go once you can grow them, since when you're growing flowers you're working not only towards High Roller, A Pretty Penny, Cream of the Crop, Flower Power and probably Green Thumb, as well as masteries for the individual flowers. The only down side of that, as far as I can tell is that (at least for daffodills, red tulips and pink roses, the return on investment of coins is not as good as for cotton (for example).