Here's a place to start:
Bike Tires Direct: Gravel Tires 700 x 40
I like a fish-scale or 'file' tread for an all-arounder, like the Schwalbe G-one, but if you ride a lot of really loose surfaces, then you could go to something aggressive like the WTB Nano, or any of the dozens in between.
Unless you're doing full sends on your hybrid, even a little bit of tread I'd going to be far better than the slicks you've got on there now