If you don't want your neighbors to hate you and you possibly run afoul of local weed ordinances, it has to be done deliberately, you can't just let the weeds grow.
Probably need to till it to cut back the weed competition and then seed with native wildflower and grasses mix. Check the Prairie Moon catalog for good seed mixes for different situations: http://www.prairiemoon.com/
They have mixes for wetlands, dry lands, sun, shade, etc. and will even custom design a mix just for your situation.
And it really helps it look planned and obviously intentional if you do a mowed path around the perimeter.