I like Marisol! I also think that May and June would work...I think of those months/names as very sunny :-). I also kinda get a sunny feeling from flower names, like Violet. Good luck!
My baby's middle name is Sunshine, so I definitely think it's a useable name. Also:
Helena-bright shining one
Beatrice sounds cheerful
I have a Saule Poppy and an Ursa Luna :)
Marigold is the sunniest of names to me. The flower even looks like a burst of sunlight. :)