For Smithing, I harvest Hides, and Improve Light Armor from Loot. It's not much Experience each time, but you can get a pile of it from every Bandit Hideout, and knock it out real quick at the first Workbench you come across. Especially Foresworn, because they're all over the Reach (With a lot of Wolves, Deer, Bears, Sabercats, and Foxes if you can hit them.) and often have Smithy stations nearby. Serpent's Bluff is centrally located, and has a Workbench, so you can sweep the Reach, fast travel back and forth, and pretty much keep adventuring. You basically don't ever have to stop for Mining, and Smelting, just shoot game as they appear, and be on your way. Once you get into Giants, they usually drop several hides as well.
Fox pelts are the lightest, while Horse/Cow/Bear pelts yield several Leathers per pelt, so those are the best for carrying around until you get to a Tanning Rack.