The line of quests that begins with finding the wrecked raft is entirely possible at the minimum level of 47, up until you have to slay an Ironwood treant in Felwood.  It can be a pain to reach Winterspring, but I did so at 46 with Goblin Rocket Boots without being killed.  It just involves a lot of travel.  But at about 6000 experience a pop, these quests have gotten me 3/4 of the way to 48.  J.D. Collie's crystal quests and a little killing have gotten me the rest.