
Second the irises :) They need very boggy ground, so best in the pond or in a boggy side-bit (if that makes sense). I have yellow flag irises in both a giant trug (with holes in) in the pond, and in a plastic dog bed at the side acting as a filter. They are thriving :)
Water lillies too, worth having other plants for variety when the flowers aren't there - I have sedge in my pond, my variety (carex riparia) is not for the little pond but there are smaller types - they have little tiny bullrush type flowerheads that look lovely.
Scirpus are another little grassy plant that look really nice - they only grow to a little over a foot if that, and form kind of a mop of fine, rich green grass, with a teeny-tiny white flowerhead on the end of the blade. Different and attractive.