Good post ! I've only got a medium-sized garden, but I've done what I can to help biodiversity along.
I make sure the garden has lots of undergrowth and wild areas; sure we get weeds, but we also get wildflowers which are bee-friendly, and the undergrowth gives the frogs somewhere to hide. Everything I plant is either edible for humans or bee-friendly.
We have to try to keep the front garden reasonably neat so the house doesn't look like a total squat. So every couple of years I put a thick layer of bark chips down on the front flower bed. I didn't plan it, but it turns out that when it mulches down into the soil, it's the perfect habitat for stag beetle larvae. They're quite rare now in the UK as a whole, but not in our garden !
One of the biggest changes I made about 5 years ago was to the roses. We had a terrible problem with blackfly and aphids. I used to spray them two or three times a year with insecticide, and it didn't really help. So one year, I decided to just give up and let nature take it's course. If we lost the roses, we lost the roses. Instead, we had all the sparrows and great tits flying into our garden, eating up the bugs and making nests. We had the best roses for years ! The birds have stayed and been joined by robins, jays and blackbirds. It's lovely looking out of my office window and seeing them all in our own mini bird sanctuary !