Yep, a willow is a good idea for your soggy bit. In amongst what I assume will become meadow, something fruiting like apple, pear or crab would be good. You might comsider some mixed native hedging too - hawthorn is brilliant for critters, an mixes well with blackthorn, holly, beech and hazel. In my experience, the odd escallonia and japanese quince in with it all extends the flowering season, and many creatures appreciate a bit of pyracantha too. If you've an opportunity, a shrubby area is nice to do. I'd include mahonia and daphne with early bulbs underneath, plus choisya, single roses, lilac, buddleia, and ground cover ivies. The odd stone or wood pile is good, as is a generous clump of nettles somewhere they wont offend you - these are the only food of many a caterpillar, so butterflies really need them. Its also worth planting some burnet in with your grass if you're letting it grow long. Burnet moths are fantastic little guys. Clover is also worth encouraging for bees, as is lavender.