Minecraft wiki entry for Building_a_metropolis, associated text: Gravel/partial cobble path:
Recommended material: Gravel, or gravel with some cobblestone mixed in
Should wind around anything that can't be altered with a shovel or axe in less than a minute, and certainly shouldn't bridge anything unnecessarily. (So bridging a small river or stream would be fine, but lakes, not so much.)
Should be 2 to 3 blocks wide.
Can be any reasonably walk-able steepness (up to 1/2 slope).