Something that involves some kind of physical activity is always awesome. One of the coolest "dates" I've been on was when a travelling fair ground was in town... nothing like bumper cars for bonding!
But really my best advice is to know your city. I think I know my city well but I'm still always discovering new gems in hidden locations I didn't know about.
Further considerations:
What are you interested in? What is she interested in? If you have shared interests that's often a winner. Also, what time will you be meeting? For how long? What kind of mood do you want?
What are your intentions for the meetup? You need some idea where you're going. From this you'll derive a lot of clues for what is best.