An ordinary garden can easily become an outdoor art gallery, and there are many ways to do it. You can paint your art, or have the neighborhood children paint it on your fence, or have some of your best friends have an art party where you all paint parts of the overall art project that you agree on.
It is just this simple, and as for cost, thrift stores are great places to find unknown artists works of arts. Don’t hesitate to look at art from trash finds. You may find some of the best art you can imagine. And don’t worry about it being outside; my art has been though rain, snow, sunshine, and it was still standing when I moved. Also, this whole fence was recycled from Craigslist.org in my area. The fellows that brought it to me installed it for me and did part of the painting of the wood. I did the rest later on.
The more art and the more variety you provide, the better it will be. So here is my former outdoor art gallery.