Go looking through the various areas of Dragonspyre for anybody who needs something done. These give the most experience. If you finish those, you might next try Mooshu.
Level 49 is enough to start Celestia. No level is high enough to do it all if you do it alone (and without Henchmen), unless you have a lot of patience to withstand repeated failure. If you stay in groups, Celestia won't be much of a problem.