Sounds like an ideal place for a pond.
Lots of good advice from Dave. As I read through this I was thinking of a pond and see that has now been suggested. That's what I would do. They add a whole new dimension to a garden
Please, if you have a trampoline in the garden, either sink it so the bed is at ground level, or net it. My friend is a radiographer. She gets to x ray an awful lot of kids who have come off trampolines.