what a neat idea, I would think herbs would be great, things you could make tea from or mix into food to feel magical! Also maybe some of those freaky fly trap plants that close in on bugs (just make sure not dangerouse obviously to kids).

I can think of a few plants that while they probably weren't mentioned in Harry Potter at all, are definitely odd and unusual for a child, and thus would probably fit fine. They could even give them their own names with some rustic looking wooden plant markers.

Allium x proliferum (Walking Onion)
Passiflora caerulea (hardy Passionflower)
Spilanthes oleracea (Toothache plant, Eyeball plant)
Tulip - Parrot varieties
Antirhinnum majus (Snapdragons - go for the Rocket varieties, they can top 3'!)
Corylus avellana (Corkscrew hazel, Walking Stick)
Stevia rebaudiana (Stevia ... sweet to chew on)
Sempervivum tectorum (Hens and Chicks)
various Sedums
Platycodon grandiflorus (Balloon flower)
Physalis alkekengi (Chinese Lantern)
Tricyrtis macropoda (Toad Lily)
Gaultheria procumbens (Wintergreen)
Best for last:
Hamamelis virginiana (Witch Hazel)
(would also suggest members of the broom family, but Scotch Broom is a pest, and none are really that odd or unusual).

Okay: so I basically took a mental stroll through the nursery I work at, and just fired off things that are 'quirky'. Some to look at, some to watch grow, and of course some to chew on. Google up a few, see what you like. Hope this helps!

King Stropharia mushrooms. Put them on the list.

I just came across this old thread and am intrigued. Did anybody plant a Harry Potter garden last year? If so, how did it work out?

One addition: Walking-stick kale. What better way for a budding wizard obtain a magic staff than by growing it him-/herself?