The canna will produce a round black seed after it flowers.

Hydrangea produces dust-like seeds, canna is as Baci described, wisteria produce about 6" bean-like pods with flattish black "beans" inside (as they are pea-relatives), and gladiolus produces pale-brown seeds with "fins" around them - they are related to irises, if I remember correctly, so the pods should be somewhat iris-like. I think hydrangeas would be the hardest to find the seeds from. I'm not sure I've collected actual seeds yet (as they are so tiny). I have seen a picture of the seeds, though.

In addition to this fine info Baci and Magus have given you, I will include a link for the seedsite which has great pictures and info:)
Happy Gardening,
Jenn

Here is a link that might be useful: http://theseedsite.co.uk/seedpods.html