You have made an investment in a piece of property and you sometimes have to think about the future. If you sell the house will the Garden of Eden be something other people would like? If they have kids, they would want a yard for the kids to play in, to put in swing sets, dogs, picnic tables, bbq equipment, etc. If they love the house but hate the yard, they will look elsewhere.
Are you interested in an in ground pool? That would take up some space but again, more work would be needed to maintain the pool.
You could put in a massive concrete patio, put in concrete sidewalks. Put in some decorative outdoor stepping stones. Put in some outdoor sculptures
How about artificial turf?
If you plant trees, put a circular 'well' around each tree and fill with mulch.
Hosta plants are nice and get pretty big. Makes a nice boarder and takes up about a 3 foot diameter when fully matured.
Put in shrubs.
Put in raised flower beds or raised veggie gardens.
Put in a fire pit.
Put in an outdoor fountain.
https://www.bing.com/videos/search?q=how+to+install+an+outdoor+fountain&view=detail&mid=811AAA962EA8AD4D88BE811AAA962EA8AD4D88BE&FORM=VIREYou can make boarders, sidewalks, flower beds of mulch. Make sure you use cardboard or landscape cloth underneath to kill the grass. This is probably the cheapest option. Each year you can add fresh mulch to brighten it all up.
Can you put in a 2 car garage with a loft on top to take up space? You could make the loft a studio apartment and get a renter to live there to make money.
Keep in mind that when you let the yard go back to nature, you may have an increase in ticks which can cause serious illness.
Like I said in the beginning, if you are going to do something to your yard that will be kind of permanent, think it over because when/if you go to sell it, back yards are very important to families typically.