I'm not sure if the margins are still there because I haven't played in a while, but I used to buy dragonstones/crystal key pieces, craft them into bolts, and enchant the bolts. It used to be quite profitable and good crafting/mage exp. Enchanting bolts is among the best mage exp in the game, so it might be worth looking into other bolts as well.
Other than that, bolting / waving metal dragons comes to mind (deaths are more expensive than bloods, so it's not a good idea to blast).