I'm a fan of the Panaracer Pasela and Pasela TG tires. Available in 25mm width, I've had great luck with them (in a 32mm) on terrain all the way up to 12+ miles of loose gravel forest service roads on a backwoods mountain pass. They have a light tread to them which is good for fire-road and grass path "off roading", but I wouldn't suggest them for anything beyond what you're comfortable subjecting a road bike to.
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"I feel like my world was classier before I found cyclocross."
- Mandi M.