Firstly - do not approach the council. This will alert them to the fact that you are planning to do something. It is possible they may be they may be friendly and helpful. But I have never found that.
My first thought is that none of the things you have considered are particularly obvious or likely to make an impact on the surrounding land. Trees, shrubs and wild flower meadows are in keeping with an agricultural use. And creating a pond in a wet corner would not be noticed., and difficult to prove you were not clearing out a pre existing pond.
As to whether you can claim the field is part of your garden is another matter. As it is accessed directly from your garden, it could be said to have become part of the curtilage of your house, over time and that would give you permitted development to put up sheds and green houses.
I have a similar situation whereby I bought a plot of land and built a house. I gradually bought more land at the back of it and now have an acre. Technically only the first plot is my garden. But no one round about is bothered, or even knows the planning laws, and so I have extended the garden, planted fruit trees, and put sheds and a greenhouse up and no one cares as it affects no one else, and is the sort of thing you expect to see in a rural area. Basically, unless you do something incredibly intrusive and upset your neighbours, then no one is going to complain to the council. The council will only investigate if someone complains.
That is how it works in practice! But if you want the cut and dried legal planning answer then you are best joining a planning forum where someone will give you the finer technical answer.