Wider tires
 

I have a Trek 7500 Hybrid w/700 35c tires. Is it possible to substitute these tires with something a bit more aggressive, more of a bike path/off road configuration? I’m not looking for MB tires, just a bit more on the ground and more aggressive tread.

If there is room physically, then yes. The chainstays are usually the limiting factor. Take a squint and see how much room you have.

If you want to go more than slightly larger, then you need to check your rims are compatible with tyre width you have chosen. See http://www.sheldonbrown.com/tire-sizing.html

But for dual road/light off road the best tyre available at the moment would is probably a 40mm Dureme. There's a review of them in the Cyclocross sub-forum. Not cheap, but fast, versatile, tough and long lasting.

Alot will depend on the surface you want to run on. For hardpack, you'll likely find your existing tires quite acceptable. On loose gravel however, they can feel more unstable, but this can also be improved with lower tire pressure. Experiment!

Good point - lower pressure handles better off road. Don't go low enough to pinch flat though!

The duremes look nice.
I have a set of schwalbe CX comp's and like them very much for both road and offroad.
They are cheaper than the duremes :)

The CX is supposed to be very nice - just not as fast or puncture proof as the Dureme, or as long lasting. Mile by mile, the Dureme may work out cheaper. Anyway - doesn't the CX only come in 35? And the OP is looking for at least a 40.

Thanks for the input.
What are your thoughts on the Marathon Extreme 40mm?

It's an excellent tyre. It's the Dureme's more off road oriented brother and has a lot more capability on soft stuff, but less on the road.