Names like Clover, Hazel, and Ruby feel like obvious companions to Maple.
For boys, I like Riley, Harrison, and Henry. And Denver is one of the few place names that doesn't grate on me (I like Dallas as well).
I second someone's Harrison Riley suggestion. I also like Henry Maxwell or Henry Gray.
I was going to suggest the Rex Harrison combination, and then I realized that felt "right" because I was thinking of the actor.
For girls, in addition to the ones above, I like Daphne, Lucy, Florence, and Ivy. Pearl feels like it would make a great middle name combination with one of those. And I love Daphne Gray.
And you've received some other fantastic suggestions:
-Gray works so well as a middle name for a lot of these, especially Harriet, Beatrice, and most of the boys' names (Scarlett Gray poses a color conflict problem).
I'd also add Winter. Winter Penelope.
Willow's lovely, but you said you were looking for unusual, so that won't work; Aspen would probably be too trendy for you too...but if Riley's popularity hasn't dulled its charm for you, then maybe one of these could still hold appeal.
Forrest/Forest (too nature-y?)
Ok, I might caution you against using another flower/ nature/ noun-y name unless you are dedicated to this trend or this is your last child. I mean, if you have a Maple and an Iris, then the 3rd daughter would HAVE to be a plant... otherwise she'd seem like the odd man out.
Favourite noun girl names: Clover, Iris
Favourite non-noun girl names: Daphne
Suggestions for girls:
Annecy (a lake)
Aurora (the lights)
For the boys, I like Denver or Harrison... Denver has the nature edge...
Suggestions for boys:
Walden (THE nature book)
Although I like Leif and Forrest, I would stay away from them with Maple (too corny)
Niall (pronounced Nile)
Caelum (Latin for sky)