Toad habitat! That's a good alternative. Toads like things to hide under and rocks to sun on, and some bare soil that's sheltered from the rain to burrow in. You could certainly do worse than make some toad
shelters around the garden. I've seen toads making holes under overturned flowerpot saucers that happened to be left propped up at one side.
There's no possibility of ducks? What a shame--a lot of people near me are keeping a few ducks in small backyards. My
permaculture teacher keeps a bathtub of rainwater and filtered greywater for them to paddle in and herds them around his garden on slug patrol. (A friend of his learned her greywater filtration system wasn't effective
enough when her ducks started sinking--if you don't get all the detergent out of the
water, the ducks don't float, which puts them in quite a predicament.)