Best wishes for a great wedding day. You might try the Deseversky Center in Old Westbury (www.NYIT.com) which has both indoor and outdoor areas. I know you want an outdoor wedding, but remember to have a Plan B in case of rain. Also check with Hofstra University which has beautiful grounds and gardens, and hosts a variety of events.
And if you need a great photographer to capture those beautiful scenes, give me a call!