If you're going for the environmental option, then....
Put in a pond - the bigger the better, but any (including a small water feature) is better than none.
Minimise the lawn area - wildflower is better than shrubs, shrubs are better than grass, a mix of both is the best option.
Replace fence with hedge - mixed natives (or, for future-proofing, mixed Mediterranean) is ideal. If you simply don't have room for hedging, grow climbers up the fence (and create cut-outs in the fence for hedgehogs, frogs etc to scurry around)
Add trees - flowering and fruiting for preference, but any tree is a haven for wildlife (and cooling the garden as a whole as we look to the future).
Keep a patch of dead stuff - logs, branches, brush as a home for all sorts of insects.
In terms of maintenance...
Ponds can be pretty high maintenance for the first few years, but there are various things you can do to reduce it (water black, algicide, solar pumps/filters/air stones).
Wildflower meadow needs mowing 2-3 times per year, and can be as easy to start as lawn (either on a roll or from seed). Shrubs need pruning once a year. If you have the space for both, do both, and make your own compost from the cuttings.
Hedging again needs pruning once a year. £150 will get you a multi-tool that can to the meadow, the shrubs and the hedge. 1 weekend is likely to be enough for any garden, unless we're talking "glorified field"
Trees need... basically no maintenance, bar leaf blowing in the autumn.
Bunch of dead stuff requires no maintenance at all once set up.
Most important of all - variety is the spice of life - any monoculture is bad for the environment.
If putting in a pathway, then stepping stones > gravel > self-binding > stone paving > concrete paving > asphalt. Ideally the same goes for seating/entertaining areas though the practicalities are different (and stepping stones not really viable).
My garden is small (6x8 metres, or 3x4 fence panels) in a modern (2003) housing estate. It started as a shitty lawn that wasn't surviving the builders' rubble, and a shitty "conservatory" that was too hot in summer, too cold in winter, and too damp to be used to storage. We took the conservatory down during first lockdown, and were left with a 6" deep slab of concrete for the first 3m out from the house.
We now have stone (slate) paving for entertaining, a gravel (slate) area that's always shaded, with planting through, a "large" wildflower meadow with a small pond and stepping stone (slate) path, largish raised beds, a small dry-stone (slate) wall-seat and a dead-stuff area (near the pond), climbers on the wall and part of the fence. I also have a home-made shed and bin storage with sedum roof and a few small trees / big shrubs mostly in pots until we find the best place for them.
Future plans include a "log store" bench, and replacing the fence with hedging when it does.
