From what I've noticed, forging things levels you up faster than improving already existing weapons and armor. So I guess you can do that or just not craft as much.
Sans, mods. this is the best method. You can improve the items you find, and sell them, thus leveling your crafting slower as well as leveling speech.
For enchanting, not much help there without a mod. You can only disenchant so many items.
Alchemy levels the slowest, and at the best pace, I've found. The problem with the others is that there are no diminishing returns. Crafting/Enchanting iron daggers is just as effective as crafting/enchanting glass, for instance.